528 STANLEY C. BALL 



9. Post-embryonic development 



In order that the further development of the sex organs, in- 

 testine, mesenchyme and glands could be studied, an attempt 

 was made to keep alive in small aquaria young worms which 

 had been removed from adults, into whose parenchyma they 

 had already become freed. No success was met with; all died 

 inside of seventy-two 'hours. 



Therefore several young of the smallest size obtainable from 

 the mussels by ordinary methods were sectioned in the hope 

 that some of the organs would be found in an undeveloped state. 

 While this trial gave some interesting results, it is desirable 

 that a study be made of specimens younger than these and at the 

 same time older than those secured at the time of birth. 



The following observations are submitted as being of interest 

 in this connection. 



a. The definitive intestine. Hallez describes and figures the adult 

 intestine of Paravortex cardii as consisting of a veritable syncy- 

 tium, a mass of cytoplasm containing large vacuoles and scattered 

 nuclei. The intestinal wall of P. gemellipara, however, consists 

 of elements which are distinct except at their extreme bases. 

 Figure 47 was drawn from a worm which measured 0.30 mm. in 

 length. The intestine, having already assumed the adult con- 

 dition, is seen to consist of large cells which extend from the outer 

 surface of the organ inward to its lumen. Their bases are con- 

 stricted and narrowed, while the distal ends assume a rounded 

 form where they extend into the intestinal cavity. Numerous 

 vacuoles, some of them containing food materials, are distributed 

 throughout the cytoplasm. Most of the nuclei are located in 

 the narrowed ends of the cells. Figures 8 and 9 exhibit a similar 

 appearance of the intestine of old worms. A like structure of the 

 turbellarian digestive tube is figured in such works as von 

 Graff's Monograph and Bronn's ''Klassen und Ordnungen," 



b. Mesenchyme. According to Hallez 's ('09) account of P. cardii, 

 the mesenchyme receives elements, both from the undifferenti- 

 ated cell mass at the anterior end of the body and from the 

 large cells which become grouped at the posterior extremity. 



