On the Rhizocephalan Genus Thompsonia, etc. 21 



It is interesting, however, to compare this organ with the so-called 

 "spadix" in the sporosacs of the Hydromedusae. Here we have an 

 exceedingly degenerate form of the medusa, with the germ cells lying 

 between a single layer of ectodermal cells and the endodermal core. 

 This latter is the "spadix" and it forms a tube which may be greatly 

 elongated and even branched, wandering in the midst of the generative 

 cells. In the general simplification of structure it is thus elaborated, 

 no doubt, to supply nourishment to the developing eggs. But the 

 organ described in Thompsonia is by no means so definite as this. 



THE LIBERATION OF THE CYPRIS LARVAE. 



In Coutiere's first note upon Thylacoplethus it is clearly stated that 

 a cloacal aperture is present, which is, however, closed by "le paroi 

 externe du manteau," by which I understand that the protoplasmic 

 part of the mantle is penetrated by an aperture, but the chitinous 

 envelope is entirely continuous. He believes that the embryos are 

 liberated by a moult. Hafele, however, denies the existence of a 

 cloacal aperture in the mantle of those species of Thompsonia which 

 came under his notice. In T. japonica he states that the larvae are 

 finally freed by a longitudinal splitting of the chitinous envelope (1. c., 

 Taf. 1, Fig. 1). But in a second unnamed species every external sac 

 possesses a genital pore that is, an opening in the chitinous envelope 

 closed by a lid which falls away at the appropriate time. The agency 

 of a moult is not required. 



An investigation of my own material leads to conclusions in harmony 

 with those of Coutiere. In one example a number of the external sacs 

 were more or less empty. All these had lost their chitinous envelope, 

 save for a frayed collar round the peduncle. A small aperture was 

 situated at the apex of the sac in the mantle and this gave a definite 

 outlet for the embryos. This is represented in text-figure 5. The 

 aperture must, however, make its appearance at a late phase of devel- 

 opment; for, although it is such a regular and definite feature here, no 

 signs of such an organ occur in any other specimens in my possession. 



It seems likely that the description which Hafele gives of the larvae 

 of T. japonica escaping by means of an irregular rent in the chitin is 

 an observation of an accidental case. With regard to the definite lid 

 observed in another species, this may constitute a true specific dis- 

 tinction. 



THE FORMATION OF NEW EXTERNAL SACS. 



When the Cypris larvae have all escaped nothing is left but a hollow 

 sac (cf . text-figure 5) . As the visceral mass thus disintegrates with the 

 progress of the development of the larvae, the question naturally arises 

 whether the existence of the parasite is terminated at the birth of 



