Their organs of generation are symmetrical, and repeated on each side of the menial line, and there are a pair of sexual orifices.
a, the animal and shell seen from above; b, the animal seen from below; r, side view of the shell sod animal In a ereeptng or adhering state; cf, portion of branchlas magnified.
The nervous system consists of what may be termed a complete cenophagean ring, and of various branches, which are given off divergingly towards the several organs.
The locomotive organs consist of an oval foot, more or less wide, according to the species, and extends the whole length of the animal.
The shell is composed of eight narrow, transverse, calcareous pieces, overlapping each other, and strongly implanted on each side in a thick and fibrous border of the mantle, which surrounds the whole body, and is sometimes, as we have observed, naked, but mere generally covered with small scales, lupines, or hairs. 'flask, pieces are not immoveable, as the animal can roll itself up or stretch itself out again for the purpose of progression or adhesion. To work this machinery, there are three muscles given off from the first piece to the second, three others given off from the second to the third, and so on throughout, no as to make the mechanism of this scale-armour complete. Oue of these muscles occupies the modal and dorsal line, the other two are lateral and oblique. Dr. J. E. Gray regards the posterior plate as the homologue of the limpet shell.
The Chitona then resemble the other Mollusks : I, in the general form of the body ; 2, in the organ of locomotion ; 3, in the nature, form, and position of the branchiie ; 4, in the heart, and in tho distribution of the circulating vessels ; 5, in the mouth and its veil; 6, in the tongue and the rest of the digestive organs; 7, in the position of the vent; and 8, last, but not least, in the nervous system.
What., then, are the differences I 1, the form of the shell composed of eight pieces instead of one ; 2, the mantle, which is more fleshy and fibrous than in the other mollusks; 3, the reyology ; 4, tho double issue of the organs of generation, allowing this difference to be established, whereas it is doubted. With regard to the absence of
eyes, that defect exists in a considerable numter of mollusks.
Professor E. Forbes and Mr. Hanley place the family Chitonidat next before the Pate/fir/tr. They say : "Aa our knowledge stands at present, we prefer to regard them as an abnormal family of Proso branehiata, and trust before long that some active observer resident by the coast will occupy himself with studying the development of the Chitons, and endeavour to ascertain the form they assume in their larval condition. Whoever does so will make an important discovery, and do more towards fixing the true position of these anomalous creatures than all cabinet examinations of them have yet enabled us to effect." Geographical Distribution.—The species are numerous, and there are few rocky shores without some of then]. Asa general rule, the largest are found in warm climates, but there are exceptions for instance, Chits.» stayer and Oaten Boweall (King), are found on the shores of Tierra del Fuego, and in the Straits of Magalhaene ; the former of these species grows to the length of 21 inches, and the breadth of 1# inches, and the latter to the length of 31 inches, and the breadth of 14 inches.
The species are found on rocky shores, where they adhere, and also on atones and other submarine bodies. They are found at depths varying from the surface to 25 fathoms. A few are found creeping on the sand.
Most zoologists are agreed that there are no differences sufficiently strongly marked to make a generic distinction between Chitots and Chi/one/fits; and, indeed, the gradations from the one to the other are no imperceptible, that there is no points where the line can be satisfactorily drawn. In the most completely-developed form of Chilon the Shelly accretion greatly preponderates ; in Chitone//as that secretion is comparatively small, and the great development is in the border of the mantle, which, in some instances, almost hides the comparatively-minute ahelly pieces.