CIRCULATORY SYSTEM OF THE GIANT SCALLOP. 239 



type that possesses at least one pair of ganglia, the pleural, that 

 are not commonly found in lamellibranchs. This view seems to 

 be based largely upon the acceptance of a hypothetical type for 

 a primitive mollusk that seems to me to be a much better ancestor 

 for the gastropods than for the other classes of the Mollusca. 



PHYLOGENY. 



The hypothetical primitive mollusk that has persistently been 

 offered for our consideration, and has found its way into a num- 

 ber of text-books, among which is Lang's " Text-Book of Com- 

 parative Anatomy," has the dorsal portion of the body covered 

 by a conical shell, the foot flattened and adapted for creeping, a 

 head fold that may be protruded from beneath the shell, a pair 

 of plumose gills, and a nervous system with at least four pairs of 

 definite ganglia, cerebral, pleural, pedal and visceral. Distinctly 

 gastropod throughout. 



If the development of animals is to be considered of any 

 importance in pointing their possible lines of descent, and as 

 long as embryo chicks have gill arches our belief has good 

 foundation, it would seem that in those mollusks whose eggs 

 are not loaded with yolk, whose embryos are not modified 

 for protection in brood pouches, and do not have long larval his- 

 tories that call for special modifications to enable them to cope 

 with enemies and to get food, the embryos might be suggestive. 



The presence of unlimited food and protection always tend to 

 destroy characters. Thus we find that parasitic forms may have 

 entirely lost organs that must have been well developed before 

 the animals took to parasitic lives. The presence of a quantity 

 of yolk furthermore frequently must have mechanical effects on 

 the developing embryo that cause direct modification. Again 

 those embryos that pass through long larval histories exposed to 

 the competition of forms that would eat their food and other 

 forms that would eat them, must necessarily be exposed to the 

 same evolutionary factors, whatever they may be, that adult ani- 

 mals are exposed to and we would accordingly expect adaptive 

 modifications in them. 



There are many lamellibranchs, and not a few gastropods, 

 that do not seem to be seriously modified by any of these fac- 



