224 



CRUSTACEA. 



dermal, and the chief constituent of which is the cell-mass of the 

 ovary. Besides remains of pigment and food-yolk, other mesodermal 

 elements no doubt enter into the formation of this second layer, from 

 which are to be derived the rudiments of the testes as well as the 

 musculature and other organs of the adult Sacculina. It is important 

 to bear in mind that the encysted sac thus produced has been derived, 

 after the expulsion of the whole thorax, exclusively from the cephalic 

 section of the Cypris larva. 



The soft body of the sac-like larva now begins to develop a small 

 fine point at its anterior end (Fig. 107 D), which protrudes into 



the inner cavity of the 

 R. ;m- „„ antenna used for at- 



tachment, and this is 

 followed by the secret- 

 ing of a new chitinous 

 envelope at the surface 

 of the soft body (second 

 moult of the Kentrogon 

 stage, Fig. 107 E). As 

 the new cuticular layer 

 thickens considerably 

 over this anterior point, 

 it forms the arrow-like 

 tube which character- 

 ises the Kentrogon 

 stage. This tube, in- 

 creasing in length and 

 becoming somewhat 

 bent, invaginates the 

 anterior surface of the 

 sac (Fig. 107 E). At 

 this stage the cast off 

 Cypris shell is either 

 only very loosely at- 

 tached to the sac, or 

 is even completely 

 severed from it. 



The protrusion of the arrow now takes place (Fig. 107 F, pf), the 

 invagination just described being re-evaginated. The arrow first 

 entert the inner cavity of the adhering antenna, and thence, con- 

 ducted by the latter, passes into the soft articular membrane of the 



myy 



flnx 



Fio. 108.— Two sections through the nucleus of a Sacculina 

 Interna (after Delaoe). A, younger stage. B, older 

 stage, am, outer mantle-layer ; im, inner mantle-layer; 

 m, mesoderm-cells ; o, aperture of the perivisceral cavity; 

 ov, rudiment of the ovary ; p, perivisceral cavity. 



