582 MONTGOMERY— MORPHOLOGY OF THE [April 24, 



the pulmonates, but that there is no homology between the outer 

 kidneys of these groups. Rabl (1879) conchuled that the outer 

 kidneys of Plaiiorbis are probably comparable with the outer kid- 

 neys of freshwater prosobranchs, but not with the Urnieren. Fol 

 (1880) maintained that the outer kidneys of Pulmonates are homo- 

 logous with the Urnieren of prosobranchs. Erlanger (1893) ^^" 

 garded all the larval nephridia as homologous with each other and 

 probably also with the head kidneys of Annelids, and distinguished 

 the following kinds: (i) Outer ectoblastic kidneys (marine proso- 

 branchs) ; (2) inner mesoblastic, and these either (0) purely meso- 

 blactic (opisthobranchs), or (b) mesoblastic with the canal at least 

 in part ectoblastic (pulmonates, pelecypods, freshwater proso- 

 branchs). Mazzarelli (1904) considered the Urnieren of pele- 

 cypods, pulmonates and freshwater prosobranchs to be homologous, 

 but the external nephridia of marine prosobranchs to be different 

 structures ; and the nephrocysts of opisthobranchs to be organs that 

 have secondarily lost their ducts and that correspond with the ex- 

 cretory cells of the Urnieren of other Mollusks. Finally Glaser 

 (1905) has given a good review of the question, and maintains there 

 are at least three distinct and dyshomologous larval excretory 

 organs ( i ) Urnieren, mesectoblastic structures of prosobranchs and 

 pulmonates; (2) Aussennieren, modified ectoblastic cells of proso- 

 branchs and pulmonates; and (3) excretion cells, those of Umbrella 

 placed near the anus ; the Urnieren are further of two kinds because 

 some of them appear to be wholly ectoblastic. 



There is so much confusion of opinion with regard to the de- 

 velopment of even the same kind of excretory organ in the same 

 species, that I fully agree with Casteel (1904) "that much more 

 work must be done upon these organs of molluscan larvae before we 

 are ready to come to definite conclusions regarding their mutual 

 relations and homologies, if such exist." There are certainly two 

 distinct kinds that may occur at the same stage in the same species, 

 and that on account of their differences in position, structure and 

 origin are not homodynamous, and these are: (i) Projecting vesi- 

 cles, wholly ectoblastic, forming part of or placed near to the 

 velum; and (2) vesicular or tubular organs placed below the ecto- 

 blast and behind the preceding, which in most cases appear to be 



