9 8 



CRUSTACEA CIRRIPEDIA 



CHAP. 



of the crab's intestine, at the junction between thorax and 

 abdomen, all the adult organs are laid down in miniature, and 

 the whole structure is surrounded by an additional sac formed 

 by invagination known as the perivisceral space (Fig. 70). 

 The young " Sacculina interna " remains in this position for 

 some time, and being applied to the ventral abdominal tissues 

 of the crab just at the point where thorax and abdomen join, or 



d.i 



FIG. 69. The mid-gut of Tnachus 

 mauritanicus with a young Saccu- 

 lina overlying it, x 2. c.t, " Cen- 

 tral tumour" of the parasite; 

 d.i, d.s, inferior and superior 

 diverticula of alimentary canal 

 of host ; n, "nucleus," or body- 

 rudiment of Sacculina ; r, its 

 roots'; x, definitive position of 

 the parasite. 



FlG. 70. Later stage in the develop- 

 ment of the "Sacculina interna." 

 x 2. b, Body of Sacculina ; c.t, 

 "central tumour"; d.i, d.s, in- 

 ferior and superior diverticula of 

 alimentary canal of host ; o, open- 

 ing of perivisceral cavity of Saccu- 

 lina ; r, its roots. 



a little below it, it causes the crab's epithelium to degenerate, so 

 that when the crab moults, a little hole is left in this region of 

 the same size as the body of the Sacculina, owing to the failure 

 of the epithelium to form chitin here ; and thus the little 

 parasite is pushed through this hole and comes to the exterior 

 as the adolescent " Sacculina externa." From this point onwards 

 the crab, being inhibited in its growth through the action of the 

 parasite, never moults again ; so that the Sacculina occupies a 

 safe position protruding from the crab's abdomen, which laps over 



