CHITONIME, a natural family of Gasteropodous affording the only known instance of a protecting shell formed of many portions, or, as they have been somewhat incorrectly termed, valves, often in contact and overlapping each other, but never truly articulated. The following cut will give some idea of the structure of this ahelly covering.
These plates are bound together by a coriaceoua border, which, as we shall presently see, is either plain or beset with bristles, spines, Sze.
The early naturalists took these shells for the peculiar armour of certain serpents, a conclusion to which they were doubtless helped by the love of the marvellous, so strongly shown in the accounts of the older travellers. By degrees the true condition of these mollusks became better known; and the opposite opinions of Linnaeus and Adanaon divided the naturalists of their age. The former arranged these shells among his Multivalves, a class entirely artificial, and like all artificial classifications comprising the most heterogeneous forms. Adanson, on the contrary, took nature for his guide, and carefully observing the animal itself, while he regarded the shell as of compara tively small importance, placed Patella and Chit on aide by side in his method. But the Linntean school long reigned paramount ; and Adanson's labours were comparatively forgotten, when Cuvier began to reform the crude state in which he found the Mollusca, and Lamarck and others aided iu the work. Cuvier, who made anato mical investigation the basis of his opinions, at once pronounced in favour of Adanson. Lamarck afterwards adopted the same conclusion, but not till he had previously placed the Chitons at the end of the Acephalous MoRuske, between Fistodasa and BaJanus. Poli, in his magnificent work on the 'Testacea utriusque Sicilize,' in giving the anatomy of a 3Iediterranean species, became a valuable ally ; for although he still retained Linne's class of 3Iultivalves, and although in his anatomical details ho said nothing of the nervous system, a branch of animal organisation essentially ne cessary to be known for assigning an animal Its true place, he demonstrated enough clearly to show that the Chiron bore no relation to the other Multivalvea of Linnaeus. M. Do Blainville
however, resting upon the generative faculty of the Chitons, prOposed, in oppo sitic n to these views of Cuvier and Lamarck, which had been adopted by almost all zoologists, to form a sub-type of Mollusks under the name of Mafentorearia, in which each of the Lin mean genera Lepas and Chilon constitutes a class; the first the Lep/Idioms, or Lrpadiera, the second the Polyplaxiphorea, or Poly p/a-rip/lora. These almost singular views of De Blainville have not pre vailed among zoologists; and Cuvier, in the last edition of the ' Regne Ani mal,' arranges the Chitona at the side of the Patelhr, forming from these two genera his small family of Cyclobranchians.
Deshayes, in the article ' Oncabrion,' in the 'Eneyclop&lie 116tho dique,' enters at large into the organisation of the Chitons, and discusses with much learning and acuteness the conflicting opinions of Cuvier and De Blainville.
The following is a summary of the structure of this family :— Digestive Organs.—No projecting head, in which the Chitona resemble the Phyllidians. No tentacula, which are replaced by a kind of veil which surrounds the mouth. Eyes, as in many other Mollusks, the Pteropoda for example, wanting. Mouth and cesopha gus furnished with a very long tongue rolled spirally and armed with horny teeth, a good figure of which is given by Poli. Loren has pointed out that the teeth, especially the central one on the lingual riband, are of great importance in determining the species. Stomach intestine, and liver like those of the other Gasteropods. Vent at the posterior extremity of the body, as in the Phyllidians, Doris, &c.
Respiratory and Circulating Organa.—The branehim of the Chitons consist of a range of email triangular leaflets placed, as in the Patella and Phyllidia, in the furrow which separates the foot from the mantle. The number and extent of the branehial lamina' are of importance in determining the species. The heart is situated poste riorly in the menial and dorsal line ; it is symmetrical, and composed of a single ventricle and two auricles.