584 SUMMARY OF CURRENT RESEARCHES RELATING TO 



capsule only six or eight develope. In the case of Purpura floridana 

 a certain number of ova, after undergoing segmentation, break down 

 and are used as food by the surviving ova. In Neritina only one 

 egg, out of a number in each capsule, segments and comes to maturity. 



The eggs of Fulgur carica are large and contain much yolk: 

 a single large polar body is formed. After dividing into two 

 and then four equal spheres, four protoplasmic micromeres are 

 separated off from the macromeres ; then four more micromeres are 

 formed, and they continue to divide. This process of micromere- 

 separation goes on till the macromeres are covered ; and even after 

 the blastopore has closed, new micromeres are formed, which give rise 

 to mesoderm : thus this layer is not due entirely to a large primitive 

 " mesoblast " as in Nassa. The stomodfeum is formed on the area of 

 closure of the blastopore. The development of the endoderm was 

 not satisfactorily made out. The author holds that this method 

 of segmentation is essentially the same as that in Hirudinea, 

 Gephyrea, Turbellaria, &c, and that all have been derived from 

 forms which had a typical segmentation, such as that seen in Pul- 

 monata and many other Gastropods ; other forms in each group having 

 departed from their original mode by the subsequent loss or addition 

 of yolk. Thus, the regular segmentation so frequently occurring is 

 not primitive, but has been secondarily induced by absence of yolk. 



In Lamellibranchs, Pteropods, and Heteropods, the formation of 

 the supraoesophageal ganglion agrees with that in the typical 

 trochosphere larva of Polygordim. In marine Prosobranchs, how- 

 ever, these ganglia arise as independent ectodermal thickenings, 

 which become later on united to one another and to the pedal ganglia. 



The apical thickening in the trochosphere larva, from which the 

 supraoesophageal ganglion is formed, is represented in others by the 

 problematic cells, regarded by Wolfson as a nervous organ ; but the 

 ganglion is not, in these forms, formed from these cells. The 

 prosobranch veliger is very highly specialized, and affords an excel- 

 lent instance of larval specialization, independent of specialization of 

 the adult. 



Nervous System and Organization of Scutibranch Gastropoda.* 

 — M. E. L. Bouvier unites under the head of Scutibranch Gastropods a 

 number of molluscs which have been placed with the Cyclobranchiata 

 and the Aspidobranchiata ; they are united by the following 

 characters : — 



1. The cerebroid commissure is very long, so that the ganglia 

 are set at the sides of the digestive tube ; these ganglia are pro- 

 duced forwards and below to form a strong ganglionic projection, 

 which is united with that of the opposite side by a subcesophageal 

 commissure ; this cord is called the proboscidial commissure. 



2. The stomato-gastric system arises from the inferior point of the 

 proboscidian projection, and forms a loop ; the two sympathetic 

 ganglia are generally widely separated. 



3. The pedal ganglia are well developed and form pedal cords, 



* Comptes Rendus, cii. (1886) pp. 1177-80. 



