78 Miscellaneous. 



seen iu Falgar and so many other Gasteropoda is essentially the 

 same as that wliich occurs in certain Hirudinea, Gephyreans, Tnr- 

 bellarians, &c., and that which is to be seen in the Lamellibranchs, 

 Annelida, and other aberrant groups can be referred to the same 

 mode ; or, in other words, that the Platyhelminths, Annelida, Mol- 

 lusca, and Molluscoidea have been derived from forms which pos- 

 sessed a typical segmentation similar to that now to be seen in the 

 Pulmonates and many other Gasteropods, many forms in each group, 

 however, having departed from the original mode by reason of 

 subsequent loss or addition of food-yolk. It will follow, as a con- 

 sequence of this idea, that the regular equal segmentation, which 

 occurs in many forms belonging to these groups, is not primitive, 

 but has been secondarily induced by the conditions under which the 

 eggs segment. The third division of the theoretical considerations 

 will treat of the mesoderm. 



The third and fourth portions of the paper will treat respectively 

 of the velum and primitive excretory organs. 



The fifth portion will treat of the development of the nervous 

 system. It will be shown that the Lamellibranchs, Pteropods, and 

 Heteropods agree in the formation of their supraoesophageal gan- 

 glion with the typical Trochophore larva of PoIf/(jonltas. In the 

 marine Prosobranchs, however, the supraoesophageal ganglia arise 

 as independent local ectodermal thickenings, which have directly 

 nothing to do with a " Scheitelplatte," and which become united 

 with each other and with the pedal ganglia later. Between this 

 arrangement and that of Pteropods &c. the Pulmonates offer an 

 intermediate stage. The problematic cells which have been de- 

 scribed by so many authors as lying in the head vesicle, and as derived 

 from the ectoderm, and which were recognized by Wolfson to be a 

 nervous organ in process of degeneration, no doubt represent the 

 apical thickening from Avhich, in the Trochozoon, the Pteropods, &c., 

 the supraoesophageal ganglia are formed. In the Pulmonates the 

 ganglia do not form from these problematic cells, which soon dege- 

 nerate and disappear, but are formed, as iu the marine Prosobranchs, 

 from local proliferations of the ectoderm. There has been an ab- 

 breviation of the development in the case of the Pulmonates and 

 Prosobranchs, and it is interesting to note that the latter group 

 presents wide differences from the other Molluscan larvae in other 

 respects also, e. f/. the excretory organs. The Prosobranch Veliger 

 seems to be very highly specialized, and affords an excellent in- 

 stance of larval specialization independent of the specialization of 

 the adult. — Johns Hoplhis University Circulars, no. 49, May 1886, 

 p. 85. 



On the Development and Minute Strusture of the Pedunculated Byes of 

 Branchipus. By Dr. Carl Claus. 



The lateral eyes of Branchipus possess an increased interest 

 because, like those of the Decapoda and Stomatopoda, they are 

 placed upon movable stalks which have only been developed iu the 



