444 



THE ANATOMY OF IXVERTEBRATED ANIMALS. 



poda^ and Prosobranchiata, occur in the Silurian formations. 

 Among the Prosohi^atichiata, the PatellklcB and the Aspido- 

 hranchia are the characteristic forms of the older formations, 

 the Ctenohranchia appearing later, and acquiring tlieir pres- 

 ent relative abundance only in the later secondary and the 

 tertiary epochs. 



The Cephalopoda. — The bilateral symmetry which is so 

 obvious in the Polyplacopliora and the Scap>hopoda is but 



Fig. 124.—^, Sepia officinalis. B, lateral view of the horny ring of an acetabulum. 



little disturbed in this group of the Odontopthora. The 

 mouth and the anus are situated in the median plane, which 

 divides the body into corresponding halves ; while the bran- 

 chine, two or four in number, are disposed symmetrically on 

 each side of this plane, as are the brachial prolongations of 

 the margins of the foot. The hagmal face of the body, how- 

 ever, is not flat, as in the mollusks which have just been men- 

 tioned, but is elongated perpendicularly to the neural face, so 

 as to form a sort of sac, invested by the mantle. On the pos- 



