98 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 
Fic. 69.—The mid-gut of Jnachus Fic.*70.—Later stage in the develop- 
mauritanicus with a young Sacci- ment of the “ Sacculina interna,” 
lina overlying it, x 2. ¢.t, ‘‘ Cen- x 2. 6b, Body of Sacculina ; c.t, 
tral tumour” of the parasite ; “central tumour”; d.i, d.s, in- 
dai, d.s, inferior and superior ferior and superior diverticula of 
diverticula of alimentary canal alimentary canal of host ; 0, open- 
of host ; ”, ‘‘ nucleus,” or body- ing of perivisceral cavity of Saccu- 
rudiment of Sacculina; 7, its lina ; 7, its roots. 
roots; «, definitive position of 
the parasite. 
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 Saceulina, 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 
