as Limnæa and Planorbis, differ in having no eye stalks, the eyes being at the base of the tentacles. They are obliged to come frequently to the surface of the water to breathe. The cephalopods stand at the head of the branch. The head is set off from the body by a slight constriction, and furnished with a pair of large, staring eyes, a mouth armed with a rasping tongue and a parrotlike beak, and eight or more tentacles or arms. The body is symmetrical, and wrapped in a muscular mantle. The shell, if present, may be internal or external (Fig. 245). The nervous system is more concentrated than in other invertebrates; the cerebral ganglia are partly inclosed in a cartilaginous cranium. All the five senses are present. The class is entirely marine (breathing by plumelike gills on the sides of the body), and carnivorous. The naked species are found in every sea. Those with chambered shells (as Nautilus, Ammonites, and Orthoceras) were once very abundant; more than two thousand fossil species are known, but only three species have been found living. I. Dibranchs. - These are the most active of mollusks, FIG. 107.-Cuttlefish (Sepia officinalis); and the tyrants of the lower tribes. Among them are the largest of invertebrate animals. They are naked, having no external shell covering the body, but usually a horny or calcareous part within. They have a distinct head, prominent eyes, horny mandibles, eight or ten arms furnished with suckers, two gills, a complete tubular funnel, and an ink bag containing a peculiar fluid (sepia), of intense blackness, with which the water is darkened to facilitate escape. They have the power of changing color, like the chameleon. They crawl with their arms on the bottom of the sea, head downward, and also swim backward or forward, usually with the back downward, by means of fins, or squirt themselves backward by forcing water forward through their breathing funnels. The paper nautilus (Argonauta) and the poulpe (Octopus) have eight arms. The female argonaut secretes a thin, unchambered shell for carrying its eggs. The squid the mantle cut open: b, branchial heart; e, eye; f, fin; g, gill; i, intestine; zb, ink bag; m, cut edge of mantle; ma, mantle artery; mc, mantle cavity; mct, mantle cartilages; pvc, posterior vena cava; s, siphon; t, tentacles with sucking disks; vs, visceral sac. ten arms, the additional pair being much longer than the others. Their eyes are movable, while those of the argonaut and poulpe are fixed. The squid, so much used for bait for cod, has an internal horny "pen," and the cuttle has a spongy, calcareous "bone." The extinct Belemnites had a similar structure. Squid have been found with a body eleven feet and arms thirty-nine feet long, and parts of others still larger as much as seventy feet in total length. 2. Tetrabranchs. This group is characterized by the possession of four gills, forty or more short tentacles, and an external, chambered shell. The partitions, or septa, of the shell are united by a tube called "siphun FIG. 109. Female paper nautilus (Argonauta argo): 1, swimming toward a by ejecting water from funnel, b; 2, crawling on the bottom; 3, coiled within its shell, which is one fourth natural size. Mediterranean. cle," and the animal lives in the last and largest chamber.30 The living nautilus has a smooth, pearly FIG. 110. Pearly nautilus, with shell bisected; one half natural size. Indian Ocean. shell, a head retractile within the mantle or "hood," and calcareous mandibles, well fitted for masticating crabs, on which it feeds. The pearly nautilus dwells in the Indian Ocean, crawling on the bottom at moderate depths; and, while the shell is well known, only a few specimens, comparatively, of the animal have ever been obtained. This grand division includes the most perfect animals, or such as have the most varied functions and the most perfect and complex organs. Besides the unnumbered host of extinct forms, Ad of typical chordates is the separation of FIG. 111. the main mass of the nervous system from the general cavity of the body. A transverse section of the body exhibits two cavities, or tubes - the dorsal, containing the cerebrospinal nervous system; the ventral, inclosing the alimentary canal, heart, lungs, and a double chain of ganglia, or sympathetic system. This Ideal plans of the branches. V, transverse section of vertebrate type; 7, the same inverted. M, transverse section of molluscous type; and Md, of molluscoid. A and Ad, transverse sections of articulate type, high and low. C, longitudinal section of colenterate type; a, alimentary canal; c, body cavity. In the other figures, the alimentary canal is shaded, the heart is black, and the nervous cords are open rings. and the face. The size of the cranial capacity, compared with the area of the face, is generally the ratio of intelligence. In the lower orders, the facial part is enormously predominant, the eye orbits are directed outward, and the occipital condyles are nearly on a line with the axis of the body. In the higher orders, the face becomes subordinate to the cranium, the sensual to the mental, the eyes look forward, and the condyles approach the base of the cranium. Compare the "snouty" skull of the crocodile, and the almost vertical profile of civilized man. A straight line drawn from the middle of the ear to the base of the nose, and another from the forehead to the most prominent part of the upper jaw, will include what is called the facial angle, which roughly gives the relation between the two regions, and the intellectual rank of the animal.31 In the cold-blooded vertebrates the brain does not fill the cranium; while in birds and mammals a cast of the cranial cavity well exhibits the general features of the cerebral surface.32 All higher vertebrates are single and free. Mammals bring forth their young alive, the young before birth deriving their nourishment directly from the mother (viviparous). In almost all the others the nourishment is stored up in the egg, which is laid before hatching (oviparous), or is retained in the mother until hatched (ovoviviparous), as in some reptiles and fishes. Of the branch Chordata there are three subbranches : Adelochorda, Urochorda, and Vertebrata. The first includes Balanoglossus, a wormlike creature regarded by some zoölogists as being related to the backboned animals, together with two other forms (Rhabdopleura and Cephalodiscus) whose affinities are less plain. The second includes the tunicates, while the great mass of the Chordata belong in the third subdivision of the branch. |