ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 17/ 



Mollusca, standing in the same relation to them as the diagram to a 

 geometrical theorum, and like it, at once imaginary and true. 



The archetype of the Cephalous Mollusca then, it may be said, has 

 a bilaterally symmetrical head and body. The latter possesses on its 

 neural surface a peculiar locomotive appendage, the foot ; which con- 

 sists of three portions from before backwards, viz., the propodium, 

 the mesopodium, and the metapodium, and bears upon its lateral 

 surface a peculiar expansion, the epipodium (Plate V. [Plate 20] fig. i). 

 The haemal surface of the archetype may or may not secrete a shell 

 upon its surface or in its interior. 



If we compare this unmodified form with the vertebrate or articu- 

 late archetype, we find that the three essentially correspond. The 

 appendicular system of the Vertebrata and Articulata is represented 

 by the epipodium in the Cephalous Mollusca (Plate V. [Plate 20] 

 figs. 9, 10, II). 



Nevertheless the differences between the three archetypes are so 

 sharp and marked, as to allow of no real transition between them. 



In the Cephalous Mollusca it is the hcBinal side of the body whicli is 

 first developed. In the Articulata and Vertebrata it is the neural side 

 which first makes its appearance. 



The archetype of the Cephalous Mollusca further differs from that 

 of the Vertebrata (and resembles that of the Articulata), in the circum- 

 stance, that while in the latter the nervous and intestinal axes are 

 parallel, in the former they decussate ; that is, the cesophagus opens 

 on the neural side, passing between the great nervous commissures. 



The molluscous archetype again differs from that of both Verte- 

 brata and Articulata in its appendicular system iep), which, when it 

 exists, never presents articulations nor anything that can be called an 

 external or internal skeleton (unless indeed the funnel-cartilages of 

 Cephalopoda be such), and which is generally altogether suppressed 

 in the adult state, its place being supplied by the foot, which, as a 

 development of the central neural region into a locomotive organ, is, 

 so far as I am aware, paralleled throughout the Vertebrata and 

 Articulata by nothing but the dorsal fin of a fish. 



In the actual forms, the symmetry of the archetype is almost 

 always disturbed by the excessive development of a peculiar region of 

 the haemal surface, into what has been termed the abdomen or post- 

 abdomen, according as it is placed before or behind the anus. 



The integument of this outgrowth, commonly modified in structure 

 and having frequently a prolonged anterior or posterior margin, is the 

 " mantle." It may or may not secrete a shell. 



The development of an abdomen (Plate V. [Plate 20] figs. 2, 3,'4, 5) 

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