64 THE CONSTITUTION OF THE GASTROPOD PROTOCONCH, 



strated, may be striking, but is no way unique, and is found 

 equally in other invertebrate groups. Osten-Sacken (41), re- 

 viewing Portchinski's publications on the larvae {I'hylem- 

 bryos) of Muscidae, comes to the conclusion that: "The won- 

 derful power of adaptation of these larvae to their environ- 

 ment in a certain measure, destroys the parallelism we natur- 

 ally expect to exist between the systematic characters of the 

 larva and imago. . . . Distantly related species belonging to 

 different genera issue from larvae almost indistinguishable 

 from each other. Again closely related and almost indis- 

 tinguishable imagos, species of the same genus differ in their 

 oviposition (size and number of eggs) and their larvae follow a 

 different law of development (as to the degree of maturity 

 the larvae reach within the body of the mother, the number 

 of stages of development it passes through)." 



It has been pointed out above that, although beyond the reach 

 of phylogenetic influences of a minor kind (in the sense that 

 they do not affect its morphological characters), the Gastro- 

 pod Phylemhryo and its N ectoconch are liable to a good deal 

 of variation in form. In fact, the larval stages of this group 

 of invertebrates, like those of many others, appear to have 

 been especially susceptible to variation under conditions 

 which do not affect the adult organism, so that there are 

 marked differences in the larvae of closely allied forms. 



This phenomenon is of such common occurrence that Alfred 

 Giard has been able to draw instances from nearly every 

 group of animals, and discusses them under the name of 

 Poecilogony (9). 



Is it possible to assign these variations of the Gastropod 

 Phylemhryo to any particular causes? 



I am not in a position to offer complete explanations of any 

 instance among the Mollusca, but certain observations which 

 have been made on members of other phyla are suggestive, 

 and perhaps capable of adaptation to instances among the 

 Gastropoda. I give the observations first, and the explana- 

 tions which they suggest for the Molluscan variations later, if 



