1914.] NATURAL SCIENCES OF PHILADELPHIA. 535 



CERTAIN FEATURES OF SOLENOGASTRE DEVELOPMENT. 

 BY HAROLD HEATH. 



The solenogastres comprise a group of worm-like organisms which 

 for a full half-century have held an unsettled systematic position in 

 zoological literature. Certain features of their organization remind 

 one strongly of the mollusks; others apparently relate them to the 

 worms; and accordingly their classification has depended upon the 

 relative importance given to these resemblances. Numerous works 

 have appeared treating of their anatonw, but up to the present time 

 our knowledge of their development has been confined to two brief 

 papers by Pruvot ('90, '92). The observations therein recorded 

 are so unique in several respects that they have influenced the 

 problem in a negative way only, making it appear that in the devel- 

 opment of these organisms we have to deal with matters not closely 

 related to other animals. It has been my good fortune to be able 

 to study a small collection of solenogastre embryos, and I shall 

 endeavor to show that as a matter of fact the development is very 

 clearlj^ molluscan. 



In a report on the solenogastres of the North Pacific (Heath, '11), 

 a species, Halomenia gravida, was described which carried about 

 twenty-five embryos, in various stages of development, between the 

 branchial folds in the cloacal chamber. These were discovered only 

 after the adult was sectioned, but a careful study of sections and 

 reconstructions has rendered the course of development fairly clear 

 from the one cell stage to the point where the mid-gut, stomodseum, 

 foot and nervous system are distinctly outlined. 



At the outset it is well to state that one of the most striking fea- 

 tures of solenogastre development is the presence of a vast test, or 

 coat of ciliated cells, which envelops the larva until the metamorphosis, 

 masks the internal structures and so distorts certain details of the 

 development that it may well have appeared to Pruvot and other 

 authors that these animals are unique. I . am decidedly of the 

 opinion that Drew ( '99) was correct in regarding the test as a modified 

 velum. It is enormous assuredly, but in its relations to other organs 

 and the fact that in several other animals it is shed at the time of 

 the metamorphosis certainly points to more than a superficial resem- 



