a ciliated crown on the anterior part of the body. The idea of an 

 analogy between the Mollusca and Annelid larva has already been 

 suggested by Gegenbanr. Still more strongly does it follow from 

 these facts, that in the Annelides, as surely as in the Mollusks, the 

 mouth-opening, with the pharynx, arises immediately after the for- 



modialely after the formation of the rudimentary pharynx arise 

 the characteristic organs of the two types : in the Annelides the 

 body segments with their appendages ; in the Mollusks the foot, 

 shell and two velar lobes." 



Salensky then compares the development of Trochus with the 

 Rotifer, Brachionus, and finds some striking analogies. His facts 



history of a rotifer. 



In the second period of development of Trochus, the true Vel- 

 iger state is entered upon. The mantle and shell are formed much 

 F .„ uo ; as in Calyptrjea. The body is now 



flattened, and the ciliated crown 

 projects very slightly. The shell 

 0) has grown considerably. 



Fig. 140, B, after Salensky, repre- 

 sents this stage. The pharynx (d) 

 arises through a tube-like invagina- 

 tion of the outer germ-layer, behind 



me behind the mbuth arises 

 a projection, which indicates the beginning of a foot (/). Within 

 the foot, as well as in the anterior part of the body, may be no- 

 ticed the middle germ-layer, which arises as a layer of cells be- 

 tween the outer and inner germ-layer. 



In the following stage the form of the larva is somewhat 

 changed. The shell begins to unroll spirally on the under side of 

 the body. The velum grows more than the middle portion of the 

 head, and the lateral lobes become larger. The operculum also 

 arises on the posterior portion of the foot. 



Coming now to the mode of development of the rulmonate 

 mollusks (fresh-water and land-snails), we find that the aquatic 

 forms undergo a complete metamorphosis, while in the land->n an- 

 there is no metamorphosis, and they are hatched in nearly the 

 same form as the adult. 



