OCCULTATIONS. Jan. 6th. 48 Tauri, 6 mag. 9h. 20m. to 10h. 24m. y Tauri, 4 mag. 11h. 28m. to 12h. 33m.-30th. ƒ Piscium, 6 mag. 9h. 56m. to 10h. 26m. OUR FRESH-WATER PLANARIÆ. BY W. HOUGHTON, M.A., F.L.S. ANYBODY who has occasionally gathered water-cress, or searched amongst aquatic weeds for objects for the aquarium, must be acquainted with certain small black or brown creatures, of an oblong form when at rest, soft, smooth, and flat, and about five lines in length and one and a half broad. These are two species, or, it may be, two varieties only, of Planarian worms. Probably the collector of objects for the aquarium sees in these animated black blotches very little to attract his attention, and he throws them aside; or if curiosity tempts him to bottle a few specimens for examination, he finds that he is able to make out very little of their structure, and sees scarcely anything to interest him in their habits; and certainly when we compare these fresh-water Planaria with other rare and exquisite forms of animal life, such a Cristatella, Fredericella or Plumatella amongst our fresh-water Polyzoa, or a Melicerta or a Stephanoceros amongst the Rotatoria, these little black dabs have small claims to beauty; nevertheless the large white species, Planaria lactea, with the pink arborescent ramifications of its digestive system, is by no means devoid of beauty, whilst the commoner black or brown kinds present many points of interest to the naturalist, both in their habits and anatomy. The Planaria, as their name imperts, are of a flattened form; the under surface of some of the species bears some resemblance to the foot of a gastropodous mollusc. In many parts of their organization the Planaria resemble the Flukes which inhabit the liver and other viscera of various animals, especially ruminants; but none of the Planaria are internal parasites, nor do they, like the Flukes, undergo a metamorphosis. They inhabit fresh and salt water, and are to be found on the leaves and stems of aquatic plants, and amongst the roots. of the Laminaria, between tide marks. On the present occasion I shall confine my remarks to the fresh-water species, of which a great number have been described as occurring in France by Duges who has published two very interesting memoirs on these animals.* In this country also, * See "Annales des Sciences Naturelles," Tom. xv. and Ton. xxi. several kinds are found, of which Planaria lactea, P. torva, Polycelis nigra, P. brunnea are common everywhere in ponds, streams, and ditches. A favourite place of resort of these creatures is within the stems of Sparganium, whence they may be readily picked or scraped off with the point of a knife and placed in a bottle of water. Difficulties of dissection long prevented naturalists assigning to the Planaria their true place in the animal kingdom; Cuvier, following Müller, Linnæus, Pallas, and Lamark, placed them amongst the parenchymatous Intestinalia, or Trematode Entozoa (as the Flukes Distoma), acknowledging at the same time their resemblance to certain species of the Hirudinide or leech family. The Planaria have affinities with both these families, and lead from one to the other. Let us suppose we have under examination the largest of the British fresh-water Planaria, viz: P. lactea; this species varies a little in colour, which is either cream, roscate, or quite white; it is from six to ten lines in length, and about two lines in breadth; we first notice the delicate arborescent form of the digestive system; we place the creature on a glass slide and hold it up to the light, in its middle part we see a milk white spot which extends linearly towards the posterior extremity; by allowing the water gradually to evaporate, the animal shows signs of discomfort, and we observe a long cylindrical tube to be pushed out from a pore slightly posterior to the middle of the body: this pore is the mouth, and the tube is the proboscis, a formidable instrument of attack in these creatures; we notice two black oculiform spots, parallel, and placed on the anterior part of the back; a little below the oral aperture we see, but very indistinctly in P. lactea, another pore which belongs to the generative system; we find the body to be slimy, very soft, and readily breaking up if not handled with great care. Let us take another specimen of the same species, and with the aid of a camel's hair pencil, place it gently in a vessel of water, and observe its locomotive powers; we see it gliding in an even and regular manner, like a limax, or slug; if we touch it, the animal twists itself in various folds, or it fixes its head portion to the vessel, and, by contracting, brings the other parts of the body along, then the posterior part is attached, and the head portion elongates and advances. In the species Polycelis nigra, P. brunnea, Planaria torva, we observe the gliding locomotion to be frequently exercised on the surface of the water, the ventral side of the animals being uppermost; none of the true fresh-water Planaria can be said to swim; but an allied marine species, Leptoplana tremellaris, progresses by flapping its sides as a Ray its fins.* When we consider how varied and energetic are the movements of the *Dugès "Annal. des Sciences, Nat." xv. 151. Planarian worms, for they can contract and elongate their bodies, fold themselves together in various forms, and unfold themselves, we naturally expect to find the presence of muscular fibres; how can such movements be explained except by the admission of the existence of a muscular system? And yet there are some animals of low organization, such as the small species of the Trematoda, which are endowed with active contractility, in whose bodies, notwithstanding, no muscular fibres have been detected. With respect to the Planariæ many observers, as de Blainville, Dugès and others, entirely deny the existence of a muscular system, except in the proboscis, and genital organs; M. Quatrefages, on the other hand, describes a sub-cutaneous plane of muscular fibre, as being recognisable in some species, and Professor Owen remarks that in the Planaria, in which as in the Tania, according to his observations, the muscular system is indicated only by the striæ on the superfices of the apparently homogeneous parenchyma, the phenomena of muscularity are strikingly displayed in the varied and energetic actions of the living animal. My own investigations have satisfied me of the existence of a sub-cutaneous plane of loose muscular fibre in the fresh-water genera, Planaria and Polycelis. A nervous system in some species has been recognised by de Quatrefages, who describes it as "consisting of two ganglions, more or less intimately united, which are situated in the mesial line, near the anterior part of the body. This double ganglion, which may be called the brain, and which is sometimes visible to the naked eye, is lodged in a special lacuna or cavity, recognisable from its transparent outline, and is seen to give off nervous filaments in various directions to different parts of the body."+ In vain have I laboured in search of the faintest indications of a nervous system in the species I have examined. The relation of the Planaria to the Flukes, as already noticed, might lead us to expect the existence of a rudimentary nervous system in them as in those Entozoa, nevertheless, I think that we must still consider its presence in the fresh-water species as a subject requiring verification. Professor Rolleston, who, at my request, kindly repeated his examination of many specimens of Planaria lactea, says, "With reference to the nervous system of this species, I have never been quite satisfied that it was a real existent thing. Leydig says he has failed to see it sometimes in the fresh-water Planaria; but that the analogy of the marine Planaria, where a nerve-system is undoubtedly present, has forced him to look for it, and that he believes that a couple of pear-shaped ganglia underlying the two eyes, and *Todd's "Cyl. of Anat." ii., p. 128. + Rymer Jones, "General Structure,” p. 146. figured by him, really exist, and are the nerve-system. At page 139 of his Handbook, he says he has not been able to see any commissure connecting these two ganglia in P. lactea. I looked into the matter in the summer, and have since my return to Oxford, on the receipt of your note, looked into it again. I had written as follows for my book in the summer: speaking of the 'anterior cœcal end of the intestine passing up between the two eyes, and underlaid in the marine genera allied to this by a nervous band, passing from the ganglia in relation with the eyes. According to Schmidt and Schultze ("Zeitschr. Wiss. Zool., x. Taf. iii. fig. 1), the same is the case in the fresh-water Planaria. Of this, however, it is difficult to convince one's self, with the semi-transparent species, Dendrocælum lacteum, which is very closely allied to this.' On further investigation I see no reason to alter this." In a subsequent letter the Professor writes that "between the cœcal ends of the intestinal tree-branches and the eyes, I saw a number of large cells with hyaline contents and a centrally-placed granular nucleus. These cells, I take it, are the nerve ganglia, loosely apposed cells making up the nerve ganglia in some of the anneloids, at least if we rank Echinodermata under this head. But I saw no commissure connecting these aggregations of cells." But whatever may be the truth with respect to a nerve-system, the Planaria are certainly sensitive, the P. lactea particularly so; they are fidgetty under condensed light, especially when applied to the head; when touched with the point of a needle, or when pierced by the proboscis of one of its own kin, as by Polycelis brunnea, the large Planaria lactea, evinces by its contortions, an undoubted sensibility. Living almost exclusively in shaded habitations, the Planarice cannot be expected to possess visual organs of much complexity, but there seems no reason to doubt that the oculiform spots, which vary from two to fifty, according to the species, do in some degree perform the function of eyes. Let us now examine the digestive system of a Planarian worm. The mouth in the true Planarice, as has been already stated, lies on the ventral surface, a little below the centre of the body; it is a circular aperture, through which the muscular contractile œsophagus, or tube, is protuded, whether for the purpose of feeding or for defecation. In form this proboscis is chiefly cylindrical, as in all the fresh-water species. In the marine Leptoplana tremellaris, it is infundibuliform, and gracefully waved. The proboscis can be protruded to a considerable length; according to Dugès, it consists of two tunics, the external one being formed of longitudinal, the inner one of circular fibres, and it is to the peristaltic constrictions of this inner tunic that the mechanism of suction is due, and the passage of nutritious particles into the ramifications of the stomach and digestive apparatus. It is most curious to watch the movements of a planarian proboscis when forcibly detached from the body of the worm; it almost seems to enjoy a separate existence for a time; it may be seen to open the orifice and swallow the pulpous particles of its own body, then by contracting, to eject them through the other end. This tube is in contact with the stomach and digestive system, and opens out into it; its basilar portion being apparently connected with the stomachal walls by a very thin transparent membrane, very easily ruptured. The digesive apparatus in the true Planaria consists of a number of arborescent ramifications; there are three principal trunks, which unite about the centre of the body and receive the particles of food supplied them by the proboscis; one of these trunks proceeds in a direct line upwards towards the head, along the mesial line of the body, and from it on either side there are numerous branched coccal appendages; the other branches immediately diverge and pass down the sides of the body, again converging towards the posterior extremity. This arrangement is very apparent in Planaria lactea, in which the cœcal appendages are most numerous; in the other species, owing to the opacity of the body, it is not so readily seen; slight pressure with the compressorium, however, reveals a similar structure. The food of the Planaria consists of infusorial animalcules, small naïd worms, the blood of which they suck, sometimes, according to Dugès, without perceptibly piercing their skins. The same observer states that animalcules of the family Cyclidina are sometimes found alive in their digestive organs. That they have, however, cannibal propensities and devour each other, I have myself frequently witnessed; this character, according to my own observations, belongs more especially to the species Polycelis nigra, and P. brunnea; it is curious to see the rapidity with which one of these fellows bores a hole into the body of one of the large Planaria lactea, and crumbles him into his component particles. The attacking enemy throws his body in a fold over some portion of its victim, and immediately begins to bore into him with his proboscis; this boring very soon reduces a part of the body of the victim into minute particles which are sucked up by the proboscis, as it makes its way along. The Planaria, like the parasite Trematoda, are destitute of an anus, the undigested portion of their food being regurgitated through the suctorius tube. The circulation in these animals is described as consisting of a mesial and dorsal canal and two lateral vessels, from which proceed in all directions, a fine cutaneous network of minute VOL. XII.-NO. VI. G G |