Pj2 The Endoparasitic Development of the Rhizocephala. 



Soon after, the colleteric glands of the ovary are cleaiiy distinguishable by their 

 regular epithelium (Piate 5 fig. 16c^). At this stage ali the adult organs are fully differentiated, 

 the ovary ramifies greatly, and in the spaces between its lobes are seen muscle fibres and 

 connective tissue cells. The testes are conspicuous as darkly staining bodies in the posterior 

 part of the visceral mass. The visceral mass of the Sacculina has attained a large size, com- 

 paratively to the little "nucleus" of undifferentiated cells from which it has sprung, and it 

 now occupies nearly the whole of the centrai tumour. It is stili invested with the two sacs, 

 i. e. internally the mantle sac and externally the perisomatic sac. Now the cloacal opening is 

 formed as a rupture of the outer wall of the mantle cavity, but the opening is as yet obstructed 

 by a chitinous piate. The body of the yonng Sacculina is now gradually pushed through 

 the wide opening of the perisomatic space, and is ready for the moult of its host which will 

 convert it from a Sacculina interna into a Sacculina externa. Delage has fully explained the 

 way in which the young Sacculina becomes external and with his description I am in entire 

 agreement. The presence of the Sacculina interna causes after a while the crab's muscular 

 and epithelial tissue, to which it is applied, to degenerate; in consequence no chitin is secreted 

 on the ventral surface of the abdomen which is in contact with the growing Sacculina interna. 

 Thus when the crab moults, a round hole is left in this region rather larger than the Saccu- 

 lina, and the latter, as it goes on growing, naturally protrudes through this aperture. At first 

 there is no chitinous ring of attachment, but when the Sacculina has grown to such an extent 

 that the peduncle, by which it is attached to the crab, completely fills up the aperture in the 

 crab's abdomen through which the Sacculina made its egress, the chitinous ring is formed 

 where the peduncle is in contact with the rim of the aperture. 



C. Orientation of the body during Organogeny. 



In the foregoing account of the organogeny I have paid no attention to the morpho- 

 logical orientation of the body relatively to the axes of the host. It is now necessary to 

 explain this interesting relation. 



During the early stages of organogeny, up to the formation of the perisomatic space, 

 the morphological axes of the parasite coincide with those of the host. This fact is repre- 

 sented diagrammatically in Text fig. 14. 



The red lines represent the intestine of the crab with its asymmetrical diverticulum 

 which enters the intestine on the morphologically left side. The dorsal surface of the intestine 

 is on the left of the figure, and the right hand surface is towards the reader, the left hand 

 surface being hidden. On the ventral surface of the intestine is placed the Sacculina interna 

 (in black). 



The antero-posterior long axis of the Sacculina is given by the mesentery (mes), the 

 anterior pole of the body being given by the nervous ganglion {gn) which is at the extremity 

 pointing to the posterior pole of the host. 



