PHYLOGENESIS IN CLASSIFICATION. 245 



■mantle, which secretes the two shells; near the mouth two 

 respiratory arms, rolled into a spiral; heart present. 



The MoUusca (proper). — The Mollusca (strictly speaking) 

 always multiply by sexual reproduction, never by budding; 

 respiratory organs either branchia or lungs; a central nervous 

 mass (brain) with three pairs of ganglia; body enveloped by 

 a thick mantle, which frequently secretes a shell of one un- 

 articulated or of two articulated valves; mouth with or with- 

 out maxillar\- appendages. 



Lamellibranchs. — Mollusca, with an unsymmetrical bivalve 

 shell, furnished with a large mantle split into two lateral 

 lobes, upon which the branchial lamellae are developed 

 equally from one part to another; the two valves of the 

 shell are united by an elastic ligament, and generally by a 

 hinge furnished with teeth and sockets; mouth and arms 

 situated between the branchia in the plane of separation of 

 the two valves; ordinarily there is a muscular foot. 



Gastropoda. — Soft animal, creeping, more rarely swimming, 

 with a robust muscular foot ; presenting a head more or less 

 distinctly separate from the trunk, and a mantle undivided, 

 which generally secretes an orbicular shell in form of a low 

 cone or shield, or spirally enrolled. 



Cephalopoda. — Head pointed, separated from the rest of 

 body ; sense organs, especially eyes, attaining high degree of 

 perfection; mouth surrounded by a crown of muscular arms. 

 Body sac -form; 2 or 4 arborescent branchia, placed in a 

 cavity formed by the mantle; shell often spiral, of one or 

 many chambers, sometimes internal, or again entirely wanting. 



While the Bryozoa and the Tunicates are scarcely above 

 the Ccelenterates, and are inferior to the Echinodermes, con- 

 sidering the differentiation and perfection of their organs, 

 the Cephalopods should be ranked, without doubt, among the 

 most elevated of invertebrate animals, and in some respects 

 they seem superior to certain vertebrates. * 



In contrast to this analytic classification of Zittel, in which 

 the definition and grouping of the organisms is based upon 

 the visible and generally conspicuous characters distinguishing 



* Zittel, " Handbuch der Pal^ontologie," vol. i. pp. 571-575. 



