ON THE PRINCIPLE OF CEPHALIZATION. 173 



Diameter. Brachiopods. Monomyarians. 



Anteroposterior. Medium. Small. 



Dorso-ventral. Small. Very large. 



Transverse. Large. Very small. 



For reason of their excessive narrowness, the greater 

 number of Monomyarians lie on the right or left valve, and 

 as their peculiar form precludes the possibility of locomo- 

 tion by the usual organ, the foot, they either remain fixed, 

 or swim freely about in the water, by violently closing 

 the ; r valves, as in Lma and Fecten. 



Among the Unionidse, the highest family in the Lam- 

 ellibranchiata, the animal assumes nearly a horizontal posi- 

 tion in crawling, though the anterior end is always the 

 lowest, and generally buried in the mud. Its embryos, 

 like Monomyarians in shape, are attached to the ovisac 

 by the dorsal margin, which is straight, as in Pecten. (Lea's 

 paper on Embryonic forms of Unionidse, Journ. Acad. 

 Nat. Sci., 2d Series, Vol. IV., plate 5). 



By their violent shutting of the valves, while in embryo, 

 they may, after birth, swim, even as Pecten swims ; at all 

 events they are said to become attached by a byssal 

 thread while young. Among the Gasteropods we have a 

 few genera attached, or fixed, as in Magilus, Siliquaria, 

 Vermetus, Spiroglyphus, Nerinsea, and Petaloconchus. 

 These are now t attached posterior end downward. In 

 Calyptraea they are in a fixed position, secreting a ventral 

 valve, upon which they rest. (It would be interesting to 

 know for a certainty which part first becomes attached in 

 Vermetus and allied forms ; their first point of attachment 

 must take place at the mouth of the tube or aperture, 

 which is really anterior and ventral). The Cephalopods 

 are free. 



Thus we have the various regions of attachment, chang- 

 ing and following in the direction indicated by the arrow 

 A, in Fig. 1. 



1st, Polyzoa : dorsal attachment. 



2d, Brachiopoda : dorsal and anterior attachment. 



3d, Tunicata: anterior. 



4th, Lamellibranchiata : anterior and ventral attach- 

 ment. 



5th, Gasteropods : ventral and posterior attachment. 



