RHIZOCEPHALA. 223 



The next stage which we may compare with the pupa of the 

 Thoracica is called by Delagb the Kentrogon stage. In this stage 

 the attachment of the larva to the body of the host (Carcinus maenas), 

 and its passage into the body-cavity of the latter are accomplished. 

 After the free Cyjpris-like larva has swum about for three or four 

 days, it seeks out a host, a young shore-crab (from 3 to 12 mm. 

 broad), and attaches itself to one of the integumental setae (bb), one 

 of the adhering antennae (1) of the Cypris larva surrounding such a 

 seta near its point of insertion (Fig. 107 G). The point at which 

 the larva attaches itself is not, as we should expect, a priori, on the 

 ventral surface of the abdomen (of the host), but seems to be in- 

 discriminately chosen. Fixation often takes place on the back of the 

 host, or on one of the legs. The next change to take place may be 

 described as a moult in which many important parts of the body are 

 cast off (amputated). First the soft contents of the adhering 

 antennae are drawn in, the apodemes (chitinous tendons of the 

 antennal muscles) being at the same time expelled from within 

 the body. These latter remain attached for a long time to the 

 envelopes of the adhering antennae, which are also retained for 

 some time (Fig. 107 Z>), as they are of importance in bringing 

 about the attachment of the larva to the host. While the soft parts 

 are everywhere withdrawn from the chitinous envelope, the thorax 

 is protruded far beyond the shell -valves and amputated in toto 

 (Fig. 107 C). This can only be accomplished by a somewhat 

 extensive rupture of the body-wall, and, through this, remnants of 

 internal organs are thrust out into the larval shell and lost. Thus a 

 great part of the pigment found in the larva, as well as remains of 

 food-yolk, are thrown out, the frontal glands and the whole body 

 musculature undergoes degeneration, and the masses of detritus thus 

 produced, together with the Nauplius eye, are now eliminated. The 

 remainder of the body left after the separation of all these organs 

 draws together to form a solid oval sac (Fig. 107 D), which soon 

 becomes surrounded with a chitinous envelope. This latter is closely 

 contiguous to the sac; only at its most anterior end, that turned 

 towards the adhering antennae, it can be noticed that the soft body 

 seems to lie naked against the inner sides of those organs. The 

 newly-formed envelope is in this region probably exceedingly delicate, 

 and very closely apposed to the inner surface of the antennae. The 

 layers into which the contents of the sac break up are at this stage 

 not at all clear. A superficial ectodermal cell-layer can, however, be 

 distinguished from the inner mass which, in all probability, is meso- 



