Page:The American Cyclopædia (1879) Volume XI.djvu/59

MALACOLOGY MALACOPTERYGIANS 51 lopment of the feeling sense, the sexual ani- mals (the same as the vascular) are either ova- rial, orchitic, or renal. In his system (see his "Physiophilosophy," Kay society ed., 1847) the first class of rnollusks (venous, ovarial animals or mussels) has the following orders : I. Protozooid mussels. II. Conchozooid mus- sels; this corresponds to the acephala, and 'is characterized by a membraneous heart with two auricles. The second class (arterial, or- chitic animals or snails) has the following orders: III. Protozooid snails or androgyni (bisexual). IV. Conchozooid snails or dicecii (with separate sexes) ; this class corresponds to gasteropods, having a membraneous heart with one auricle. The third class (cardiac, nephritic animals or kraken) has the following orders: V. Protozooid kraken. VI. Concho- zooid kraken. It will be seen from this sys- tem that the principles of Cuvier respecting the different plans of the four great divisions of the animal kingdom are entirely set at nought ; orders, according to Oken, represent- ing in their respective classes the characteristic features of the lower types. Among the em- bryological systems may be mentioned those of Von Baer, Kolliker, Van Beneden, and Vogt. Von Baer (1827-'8) calls the mollusks the massive type, as the body and its parts are formed chiefly in round masses, the shape un- symmetrical, the nervous ganglia diffused and appearing late, and the movements slow and feeble ; in the course of development identical parts are produced, curving around a conical or other space. According to Kolliker (1844), in the mollusks the embryo arises from a primitive part, grows uniformly in every di- rection, and either entirely encloses the embry- onal vesicle, early in gasteropods and acephala, or late (forming a temporary vitelline sac) as in Umax, or else contracts above the embryonal vesicle, forming a genuine vitelline sac, as in cephalopods. Van Beneden (1845-'55) places mollusks with worms and radiates under his group of allocotyledones or allovitellians, in which the vitellus or yolk enters the body neither from the ventral nor from the dorsal side; his class mollusca, at the first date, in- cluded cephalopods, gasteropods, pelecypods, and brachiopods ; in his later work he added acephala, tunicata, and bryozoa, removing the last two from the polyps; the cephalopods, however, are not allovitellians, and any classi- fication which unites in one group mollusks, worms, and radiates cannot be founded on cor- rect principles. Vogt (1851) adopts the dis- tinction of Kolliker, of animals in which the embryo is developed from the whole yolk, and those in which it arises from a definite part of it, in the former of which he places mollusks, with worms and radiates ; he makes a primary division of the cephalopoda, in which the yolk is cephalic, with a class of the same, with the orders tetrabranchiata and dibranchiata. In the division mollusca, with an irregular dis- position of the organs, he makes the follow- ing classes: cephalophora, acephala, tunicata, ctenopkora, and bryozoa. The last three classes constitute his mollvscoidea. The separation of the cephalopods is unjustifiable, and the transfer of the ctenophora from acalephan radiates to mollusks cannot be maintained. Milne-Edwards (1855) divides the third branch, malacozoaria or mollusca, into the two sub- branches : 1, mollusks proper, with the classes of cephalopods. pteropods, gasteropods, and acephala; and 2, inolluscoids, with the classes tunicata and bryozoa. Agassiz, in his "Essay on Classification" (1857), makes only three classes of the branch of mollusks : I. Acepha- la, with orders : 1, bryozoa (including the wr- ticellce) ; 2, brachiopoda ; 3, tunicata ; and 4, lamellibranchiata. II. Gasteropoda, with orders: 1, pteropoda; 2, heteropoda; and 3, gasteropoda proper. III. Cephalopoda, with orders: 1, tetrabranchiata, and 2, dibranchiata. He includes bryozoa among mollusks, uniting with them the vorticellidce, 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 ascid- ians pedunculated young, resembling boltenia, and forming a connecting link with the com- pound 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 segmenta- tion of their yolk, than can the mammalia from other vertebrates on account of its total seg- mentation in their case. According to Prof. Owen, some of the compound ascidians have certain affinities to the zoophytes ; some of the marine apneusta (like actceon and glaucus) 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 for- mer including the polyzoa, tunicata, and bra- chiopoda, the latter the lamellibranchiata, gasteropoda, pteropoda, and cephalopoda. Prof. Morse places the brachiopods among the worm- like articulates ; and very likely the tunicates and polyzoa belong with them. (See BRA- OHIOPODA.) MALACOPTERTGIMS, a division of fishes es- tablished 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 at- tached under the pectorals, the pelvis being suspended to the scapular arch, comprising fishes like the cod, flounder, turbot. &c. ; 3,