Milne-Edwards (1855) divides the third branch, malacozoaria or mollusca, into the two sub-branches: l,mollusks proper, with the classes of cephalopods. pteropods, gasteropods, and acephala; and 2, molluscoids, with the classes tunicata and bryozoa. - Agassiz, in his "Essay on Classification" (1857), makes only three classes of the branch of mollusks.
Acephala, with orders: 1, bryozoa (including the ror-ticellm); 2, brachiopoda; 3, tunicata; and 4, lamellibranchiata.
Gasteropoda, with orders: 1, pteropoda; 2, heteropoda; and 3, gasteropoda proper.
Cephalopoda, with orders: 1, tetrabranchiata, and 2, dibranchmta. He includes bryozoa among mollusks, uniting with them the vorticellidtv. the plan of their structure not being radiated, but distinctly bilateral, and gradually leading through the brachiopods and tunicates to the ordinary acephala; tunicata show in the simple aseid-ians pedunculated young, resembling boltenia, and forming a connecting link with the compound ascidians; cephalopods are homologous with other mollusks in all their systems of organs, and can no more properly be separated from them on account of the partial segmentation of their yolk, than can the mammalia from other vertebrates on account of its total segmentation in their case. According to Prof. Owen, some of the compound ascidians have certain affinities to the zoophytes; some of the marine apneusta (like actaon and glaums) are related to some of the abranchiate annelids; though cephalopods are the highest, they do not pass into amphioxus or any other embryonic form of vertebrate; he retains the bryozoa with the polyps. Prof. Huxley makes the primary divisions of molluscoids and mollusca; the former including the polyzoa, tunicata, and brachiopoda, the latter the lamellibranchiata, gasteropoda, pteropoda, and cephalopoda.
Prof. Morse places the brachiopods among the wormlike articulates; and very likely the tunicates and polyzoa belong with them. (See Bramalacopterygians, a division of fishes established by Artedi in the early part of the 18th century, including such as have the fin rays soft, except occasionally the first of the dorsal or pectorals. Cuvier divided them into three orders: 1, the abdominal, in which the ventrals are suspended to the under part of the abdomen, behind the pectorals, and not attached to the scapular arch, comprising the greater part of fresh-water fishes, as the carp, pike, cat fish, salmon, herring, and their allies: 2 the subbrachian, having the ventrals attached under the pectorals, the pelvis being suspended to the scapular arch, comprising fishes like the cod, flounder, turbot, etc.; 3, the apodal, wanting ventres and sometimes the pectorals, including the eel family. J. Mailer limits the term to the group scombere-socida of the suborder pharyngognathi, including the flying fish.
This is rejected by Van der Hoveven, who returns to Ouvier's divisions, adding, however, a few families. (See Fishes, and Ichthyology).