299 



in masses of jelly on sea weeds. The morula, or mulberry mass, 

 forms as usual. The blastoderm arises from a few small 



itive dark morula cells. The four vitelline or primitive cells, 



as the blastodermic" cells. The egg now becomes flattened at 

 one pole and slightly pointed at the other, the latter being the 



In six hours after development begins, the outer layer begins 

 to form, and the first organ to arise is the velum, which at first 



bryo. This stage (Fig. 140, A, v, velum, after Salensky) is equiv- 

 alent to the trochosphere (Lankester) of the pond snail. It will 

 thus be seen that the development of Trochus is now very dif- 

 ferent from that of the Calyptrsea, where the velum, head-vesicle 

 and foot arise simultaneously. A little later the mouth and cosoph- 

 agus ri se. Salensky remarks that the Prosobranchiate Gastropods 



to the observations of Lacaze-Duthiers. the velum does not arise 



n < lypti i 4 .. \ « run tus uid V, u . imim md Purj ui u wl i 

 develop like the former mollusk, having a similar larval heart and 

 primitive kidneys, though the mode of development of the exter- 



era of Prosobranchiate Gastropods which develop as in < 

 all belonging to the suborder Ctenobranchiata. 



On the other hand. Paludina vivipara, tferitt'nafuviatiUs, and 

 certain Pteropods (Tiedemannia ncapolitana, Covolinia gibbosa) 

 and a Heteropod (Pterotraehea) are provided, as in Trochus. with 

 a ciliated crown, the first organ lying behind the primitive mouth. 



"A good starting point," adds Salensky, whom we have in reality 

 been quoting all along, "for the comparison of the development of 

 Trochus and allied forms, with that of other animals, consists in 

 this stage (Fig. 140, A). A cursory glance at the illustration, will 

 convince one that this condition of the Trochus embryo is simi- 

 lar to the larva of some annelides. Examples of such Annelid 

 larva? may be seen in some Sabcllidae (e.g., Dasychone luculbnxi) or 

 Spio (S.fuliginosus). The latter in escaping from the egg have 

 a more or less oval body consisting of two layers, its only organ 



