THE CIRRI P EDI A 



'35 



retracted' and enclosed in a new cuticle, remaining connected only 

 with the antennule which is fixed to the host (Fig. 82, D). Within 

 the sac thus formed, a tubular chitinous organ known as the " dart " 

 becomes differentiated. At first the dart is invaginated into itself, 

 and in connection Avith it a second cuticular sac becomes separated 

 within the first. The larva at this stage was designated by Delage 

 the "Kentrogon" (Fig. 82, E). The point of the dart lies within 



Sections through successive stages in development of the "nucleus" of the fran-iiHim 

 interan. <nn, outer layer of mantle; im, inner layer of mantle; m,, mesoderm cells; o, 

 aperture of inva^ination of perisomatie cavity, not shown in B; ov, rudiment of ovary ; p, 

 perisomatic cavity. (After I)i j la,u>-, from Korschelt and Heider's Embryology.) According 



ti> G. Smith, the mantle-cavity is already established when the inva.yination to form the peri- 

 .somatic cavity takes place as in A. 



the fixed antennule, and when fully formed it becomes evaginated 

 and forces its way through the cuticle of the host (Fig. 82, F). 

 Through it the contents of the sac, consisting of a mass of 

 undifferentiated cells surrounded by an ectodermal layer, pass into 

 the body-cavity (haemocoel) of the crab, and become what Delage 

 called the Sacculina internet. Probably the mass of cells is at 

 first carried passively by the blood-currents of the host, but it 

 ultimately becomes attached to the lower surface of the intestine 



