50 The Endoparasitic Development of the Rhizocephala. 



of the adult are already laid down in miniature, and fig. 4 shows that the body of the Pelto- 

 gaster is already beginning to gain access to the exterior, and yet no invagination has begun 

 to forni the rudiment of a perisomatic cavity. Fig. 6 is a section through a young Pelto- 

 gaster externus and here the method of communication of the external part of the body 

 with the centrai tumour is further proof that no perisomatic cavity has been formed, because 

 in a young Sacculina externa the ragged edges of the perisomatic cavity are always to be 

 observed instead of the clear-cut continuity of external body and internai centrai tumour shown 

 in Piate 6 fig. 6. 



With regard to the rest of the organogeny of Peltogaster I will not enter into details, 

 because it evidently follows the essential lines described for Sacculina, as may be gathered 

 from Piate 6 fig. 5, from which also the difference in shape of the young parasite can be 

 observed. 



There is however one important consideration with regard to the manner in which 

 Peltogaster internus becomes external. We have seen in Sacculina that this is always effected 

 by a moult of the host at the criticai period when the Sacculina is ready to be evaginated. 

 Now in Peltogaster there is very strong evidence to show that the parasite pushes its body 

 through the abdominal tissues of its host and so becomes external without any special moult 

 on the part of the latter. In Piate 6 fig. 4 the young parasite is already beginning to pro- 

 trude through the external tissues of its host's abdomen (red in figure), and this crab showed 

 no sign of having recently undergone a moult. The fact that the Peltogaster may become 

 evaginated from its host's body without a moult on the part of the crab has since been 

 confirmed on several occasions by Mr. F. A. Potts, whose investigations on the relation of 

 parasite to host will, I understand, be shortly published. 



Recapitulating the chief points in the endoparasitic development of Peltogaster we 

 may state : — 



1. A completely embryonic undifferentiated stage is passed through in the host's body 

 corresponding to the youngest Sacculina interna migrans. 



2. The centrai tumour does not so actively change its position as in Sacculina, after 

 once it has begun to throw out roots ; but its method of grò wth resembles that of Sacculina 

 in that the oldest roots are always further away from the point where the external body will 

 be differentiated. 



3. No perisomatic cavity is formed. 



4. The parasite probably gains to its external position independently of a moult on 

 the part of its host. 



The last three points confimi our opinion developed in Chapter 1 that Peltogaster is 

 more primitive than Sacculina. 



