126 BIOLOGICAL LECTURES. 



In Lepidonotus it is composed of sixteen cells, which arise 

 exactly as in the annelids just described (see Figs. 6-n, 

 stippled cells). These cells put out cilia very soon after they 

 are formed, as they do in Amphitrite (Fig. 10). Later, three 

 of the gaps between the separated portions of the primary pro- 

 totroch are filled by other ciliated cells, while the mid-dorsal 

 interruption persists just as in Fig. n of Amphitrite. It is not 

 improbable, therefore, that the definitive prototroch of this 

 worm has the same origin as that of other forms. Lepidonottis 

 belongs to the Aphroditidce ; the eggs are comparatively small, 

 and the cleavage is equal, so that up to the 64-cell stage 

 it is not possible to distinguish one quadrant from another. 

 This fact makes it of especial interest that the gaps in the 

 primary prototroch are filled in only three quadrants instead of 

 in all four, for, if the destiny of the secondary trochoblasts were 

 dependent only upon their position in the cell complex, we might 

 expect that the destiny of the corresponding cells would be the 

 same in all four quadrants. 



Podarke, which has been described by Treadwell, has a func- 

 tional primary prototroch of sixteen cells of the same origin 

 as in the other forms just described. (Mr. Treadwell has not 

 yet published the history of the secondary prototrochal cells.) 

 Podarke belongs to the family Nereides. The egg is even 

 smaller than that of Lepidonotus > and the cleavage is equal. 



St/ienelais y family Aphroditidce, according to Treadwell, also 

 has an equal cleavage, and a functional primary prototroch of 

 sixteen cells is derived exactly as in the five annelids which we 

 have already described. 



We may now proceed to those annelids in which the mode of 

 origin of the primary prototroch appears to be a modification 

 of the type. Among these forms Hydroides is of special 

 interest. 



Wilson and Treadwell have shown that the functional proto- 

 troch of Hydroides consists of eight cells, two in each quadrant ; 

 but the origin of the primary prototroch in Hydroides is different 

 from that of Amphitrite only in this respect, that the primary 

 trochoblasts divide only once instead of twice. In the seven 

 annelids which we have considered, the primary trochoblasts 



