but this is apparent rather than real. If the ciliated cells that 

 cover these larvae and form the tests, were pushed forward and 

 the stomodaeum shortened so the mouth would retain its position 

 at the margin of the ciliated area, the two larvae would be essen- 

 tially alike. At a slightly earlier stage in Pecten a large part 

 of the surface is covered with cilia, and this is changed only by 

 the posterior development of the embryo, beginning with the 

 flattening of the shell gland. Such a posterior development is 

 normal in many trochophores as in the case of Dondersia (23), 

 Dentalium (17), Chiton (10 and 16), and most lamellibranchs 

 and gastropods, as well as in annelids, where the posterior devel- 

 opment is so marked. 



The shell gland spreads out laterally and forms the lobes of 

 the mantle which secrete the shell valves (figs. 35 and 36). The 

 ciliated area grows rapidly and forms the two lobes of the velum 

 (vl.). The cerebral ganglon (eg.) are formed near the apical 

 plate. The alimentary canal grows dorsally and is bent into the 

 shape of a U. The stomach (s.) enlarges, the intestine (i.) 

 acquires an anal opening, the greater part of the schizocoele 

 becomes filled with mesoderm and the embryo assumes the form 

 of a veliger. This change is accomplished inside of three or 

 four hours, so active veligers are formed in about thirty hours 

 after the eggs are laid. The shape of the embryonic shell is 

 quite characteristic for the embryonic shells of lamellibranchs 

 (12) and differs very greatly from the adult form. Risser reports 

 that this is not the case with Pecten irradians (24) but the shells 

 on very young embryos that I have reared are very similar. 

 Until this stage is reached the embryos take little 

 or no food. They now swim about actively through the 

 water, frequently going to the surface. The cilia on 

 the edges of the lobes of the velum are the means of 

 locomotion. The apical cilia remain bunched and are moved 

 rather gently in different directions but apparently func- 

 tion as sensory rather than locomotary cilia. Each individual 

 occasionally retracts its velum between the valves of its shell, 

 closes its shell, and slowly settles to the bottom. This is almost 

 always the case whenever the animal is disturbed, as by jarring 

 the dish in which the veliger is swimming, or when the animal 

 runs into anything or is run into by another animal. In such 



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