628 GEOFFREY SMITH. 



tion of the cells into distinct layers or organs was present. I 

 therefore suggested the term ''embryonic cells" to designate 

 the cellular contents of the Kentrogou. After observing in 

 several instances the formation of the Kentrogon larva, it 

 appears to me that the ectoderm of the Cypris is undoubtedly 

 included in the Kentrogon together with the mesodermal 

 cells, as Delage originally maintained. At the same time, 

 his designation of the mesodermal mass of cells as the ovary 

 is obviously a misnomer, since this mesodermal mass gives 

 rise to all the mesodermal organs and tissues of the adult, 

 and, at the early stages of the endoparasitic development, is 

 quite undifferentiated (see 2, pp. 47 and 55). 



We may, therefore, retain the term "embryonic cells" for 

 the cellular contents of the Kentrogon, w,ith the admission 

 that these embryonic cells are composed of embryonic ecto- 

 derm and embryonic mesoderm. It may be remarked that, 

 in this manner, the Kentrogon of the Rhizocephala almost 

 exactly corresponds to the post-nauplius stage of Moustrilla, 

 as described by Malaquin (3), which, on penetrating the 

 ectoderm of its host, loses all the larval organs, and consists 

 of a little chitinous sac containing embryonic ectoderm and 

 meso-endoderm, from which the adult organs are regenerated. 

 In Monstrilla, of course, the endoderm is not entirely sup- 

 pressed, as is the case in the Rhizocephala, though it is on 

 the way to becoming so. 



This interesting parallelism between the Rhizocephala and 

 the Monstrillidse has not been sufficiently insisted on. 



In the extreme anterior region of the Kentrogon, where it 

 is attached to the base of the crab's hair, I have been able to 

 observe the formation of the dart which gives the name to 

 this larval stage {Kivrpov, dart; yovog, larva). This hollow 

 dart, as Delage described, elongates and pierces the base of 

 the hair, thus opening a passage through which the embry- 

 onic cells of the Kentrogon can pass into the ha)moccel of 

 the host. The dart is continuous with the inner cuticle of 

 the Kentrogon larva, and is presumably a product of the 

 ectoderm. 



