460 STANLEY C. BALL 



atrial surface and its opening into the latter is strongly con- 

 stricted by a sphincter muscle; the openings of the shell glands 

 in P. cardii are distributed along the entire ventral wall of 

 the atrial canal and antrum femininum, while in Linton's species 

 they all open at the anterior end of the antrum just back of 

 the atrium; the vitello-oviducts of P. cardii are the longer. 

 Linton's species lives as a commensal in the mantle cavity of 

 the ribbed mussel, Modiolus demissus; P. cardii is parasitic 

 in the stomach of Cardium edule. 



Linton's species resembles Paravortex scrobicularia rather 

 than P. cardii in the form of the ovaries, i.e., they are elongated 

 in the first two and shorter m P. cardii. In form of antrum femini- 

 num and lack of an atrial canal Linton's species and P. scrobi- 

 cularia are similar, but the latter, on the other hand, is simi- 

 lar to P. cardii rather than to the American form in that the 

 atrium does not project dorsally beyond the opening of the 

 antrum. 



Since Linton's species obviously differs from the European 

 forms, his specific name gemellipara is to be retained. Its 

 applicability is owing to the fact that ninety-five per cent of 

 the capsules contain two embryos. 



2. Kistorical 



While the anatomy and taxonomy of the Rhabdocoele Tur- 

 bellaria have received considerable attention, only a few works 

 have appeared dealing with their embryology. Hallez ('78) 

 published the results of his observations upon the winter eggs 

 of various Turbellaria. Since the young are enclosed within 

 a hard-shelled capsule, the material was so difficult to study 

 that he characterized these results as insufficient. In 1887 

 he did further work upon the fresh water Dendrocoeles. 



In 1903 Caullery and Mesnil described the development of 

 certain species of Fecampia, Rhabdocoeles parasitic in certain 

 Crustacea. 



One of the most complete and noteworthy contributions to 

 the embryology of this group is that of Bresslau. In 1904 he 

 published the results of his research, begun in 1898, upon the 

 embryology of four species of the family Mesostomidae, namely 



