Mollusca propria Lamellibranchiata (Mussels, Cockles, Oysters). Gasteropoda Sub-class. {Branchiogastropoda (Whelks, Periwinkles, Sea Slugs). Pteropoda ("Butterflies of the deep "). (Slugs, Snails). SUB-KINGDON-VERTEBRATA. Class. Order. Urodela (Siren, Salamanders). Anoura (Frogs, Toads). Ophidia (Snakes). Lacertilia (Lizards). Odontolex-Hesperornis. Odontotroma-Ichthyornis. 5 Ornithorhychus. Genus. Rhizophaga (Wombat). Poephaga (Kangaroos). Carpophaga (Phalangers). (Sarcophaga (Thylacine, Dasyurus). Family: Rhinoceridæ. Species. Variety. Race Horse. Equus...... ..Equus Caballus. Brewer's Horse. dactyla Equus Asinus. Equus Zebra. Camelus Dromedarius. Camelus Bactrianus. dactyla Roebuck. Red Deer. Fallow Deer. Camelopardalidæ (Giraffe). Sub-genus. Antelopidæ. Caircornia Ovidæ (Sheep and Goats). Bos Bison (Bison). Bovidæ Bos Taurus, &c. (Common Ox). * Birds with teeth are arranged in the above two Orders. The animals lowest in the scale of organisation, which belong to the class Gregarinide and to the sub-kingdom Protozoa, are devoid of mouths and digestive apparatus. They live entirely by imbibition or osmosis ; and in the adult state they have not the power of emitting pseudopodia. The animals may be found in the intestine of the cockroach, earth-worm, &c. In the next class, the Rhizopoda, are animals which possess the power of throwing out processes of their substance, as white blood-corpuscles do, and of exhibiting distinct amaboid movement. These processes are called “pseudopodia” or false feet. There is neither mouth nor anus; but by means of the pseudopodia becoming attached to and encircling nutritive particles, and drawing them into its substance, does the animal perform the function of ingestion or feeding. Any part of the surface is capable of performing this function. When a particle of food has passed into the body the aperture by which it entered immediately closes up, and the discharge of solid excreta is effected in a similar manner, except that only one portion of the general surface appears to be limited to this excretory purpose. In the substance of many of these lowly creatures nothing is to be discerned but a mass of jelly-like substance resembling a particle of thin glue. As we ascend the scale of organisation and come to the higher class of Protozoa—the Infusoria—there is to be found a month, a rudimentary digestive cavity, and an anus. The mouth leads into a funnel-shaped gullet which opens into the soft central mass of sarcode. The food passes into this body substance, and not into any definite stomach, where it is digested. The anus is situated close to the mouth, but is only seen when in use. In the sub-kingdom Cælenterata, the members of the lower class--Hydrozoa - which contains the fresh water polyps, possess a permanent mouth and body cavity, the mouth communicating with the body cavity. In the higher class Actinozoa, which includes the sea-anemones, so common in aquaria, there is, in addition to the body cavity, a distinct stomach. The mouth leads into the stomach, which is a wide membranous tube, opening by a large aperture into the body cavity, but there is no distinct alimentary canal nor anus. A Hydrozoon or Hydra, is essentially an open-mouth saccule, consisting of two membranes, an outer or ectoderm, and an inuer or endoderm. The majority of these animals seize their prey by means of tentacula, which are processes of those membranes developed either around the mouth or from the walls of the digestive cavity. a a The third sub-kingdom of Invertebrata-the Annuloidais distinguished by the animals possessing an alimentary canal, which is entirely shut off from the body cavity. In many members there is an intestine and a distinct anus. The mouth of the sea-urchin is surrounded by a series of calcareous pieces, known as the “oral plates," while a similar series of " anal plates” encircle the anus. The oral plates consist of five sharp rod-like teeth, which perforate triangular pyramids, and form a singular structure known as “ Aristotle's lantern.” The microscopic Rotifera, or wheel animalcules, have a complicated horny masticatory apparatus, with upper and lower jaws. Included in the next sub-kingdom- Annulosa-are the leeches, lobster, insects, &c. The medicinal leech has its mouth surrounded by three crescentic jaws, the convex surfaces of which are serrated with minute teeth. In the crab and lobster some of the limbs are modified so as to form maxillæ and maxillæpeds, the animal, so to speak, masticates with its legs. These are parts of the exoskeleton, or hard, subcalcareous, or chitinous crust, which protects the body. In the lobster one pair of maxillæ are finely dentated, giving the appearance of teeth; the other maxillæ have large tubercles upon them, but they are analogous to the fine dentations on the other pair of jaws. The mouth is provided with two teeth for the comminution of food. The insects may be divided generally into two groups, the masticatory and the suctorial, though both types may be modified and occasionally combined. The masticatory and suctorial apparatuses are modifications of the exoskeleton, similar to the maxillæ of the lobster. In many insects, the beetle for example, the alimentary canal is highly differentiated, being divisible into mouthi, æsophagus, crop, gizzard, stomach, intestine, and anus. The "chewing jaws” in the mouth of the beetle are minute representatives of the nippers of the lobster. Indeed, the mouths of many insects are very complex. Passing to the sub-kingdom Mollusca, in the lowest classPolyzoa, the sea-mats, &c.--the mouth is surrounded by a circle of tentacles, and each tentacle is fringed with long and active vibratile cilia, which lash water towards the mouth. The mouth leads into a long and wide pharyngeal and æsophageal tube, which opens below into a definite stomach. From this is continued a distinct intestine, which bends upon itself towards the oral end of the body, and then terminates upon the outer surface near the mouth. In the higher group of Mollusca, in the class Branchiogasteropoda—which contains the whelks, periwinkles, sea a slugs-and also in the class Pleurogasteropoda-snails, slugs, &c.—the cavity of the mouth is invariably provided with an organ denominated the “ odontophore" or " lingual ribbon.” It consists essentially of a cartilaginous cushion, supporting, as on a pulley, an elastic strap, which bears a long series of transversely disposed teeth. The ends of the strap are connected with muscles attached to the upper and lower surface of the posterior extremity of the cartilaginous cushion; and these muscles, by their alternate contractions, cause the toothed strap to work backwards and forwards over the pulley formed by its anterior end. The toothed strap consequently acts like a chain saw upon any substance to which it is applied; and the resulting wear and tear of its anterior teeth is made good by the incessant development of new teeth in the secreting sac, in which the posterior extremity of the odontophore is lodged. The substance of these teeth is merely chitinous. In certain other Mollusca there is also a differentiation of the mucous membrane of part of the alimentary canal to the extent of producing from the walls of the stomach calcareous plates for the trituration of food. Now, if we pass from the primary division Invertebrata, of which I have given examples, to the Vertebrata, we shall therein find the differentiation of tissues for dental purposes of much higher types of complexity as regards form, structure, and distribution. When an invertebrate animal possesses organs of mastication, these are either hard productions of the alimentary mucous membrane, or are modified limbs. In no vertebrate animal, on the other hand, are limbs so modified and functionally applied, the jaws being always parts of the walls of the head specially metamorphosed, and totally distinct in their nature from the limbs. If we consider the teeth of the lowest class of vertebrata -fishes—we shall find, not only as regards substance, but as to their number, form, structure, situation, and mode of attachment, that they present more various and striking modifications than do the teeth of any other class of animals. The Pharyngobranchii, or lowest order of fishes, includes only the Lancelot. This fish has no skull nor jaws, the mouth being surrounded by a cartilaginous ring composed of many pieces. The next order, Marsipobranchii, includes the Lampreys and Hag-fishes. These animals have a cartilaginous skull, but no mandible or lower jaw (Huxley), and are destitute of true calcified or dentinal teeth, the armature of the mouth consisting of horny cones or serrated plates. The labial 1 |