136 



THE CRUSTACEA 



immediately behind the stomach. It now begins to send out 

 processes which branch to form the absorptive roots, while the 

 main part of the embryo grows backwards along the intestine 

 towards the point at which emergence of the adult Sacculina 

 takes place. A thickening near the growing border forms 



FIG. 84. 



Later stages in the development of SaccHlirut. A, Sdwulina internet fully formed. B, 

 SacculiiM externn 'list after emergence from the body of the host, a, genital atrium ; am, 

 outer layer of mantle ; B, basal plate ; b, mantle-cavity ; C, central tumour ; cl, opening of 

 mantle cavity ; D, intestinal wall of host; dr, colleteric gland;/, opening of perisomatic 

 cavity' g, nerve-ganglion; im, inner layer of mantle; L, integument of host; ov, ovary; p, 

 perisomatic cavity ; pe, ectoderm of visceral mass ; R, root-processes ; /, testis. (After Delage, 

 from Korschelt and Heider's kmbryology.) 



the first rudiment of the organs of the adult, and is known as 

 the " nucleus " (Fig. 83). Within this the mantle- cavity arises by 

 delamination, and an outer cavity concentric with it, the "peri- 

 somatic " cavity (p), is formed (according to G. Smith) by an in- 

 vagination of ectoderm. When the young Sacculina is completely 

 formed (Fig. 84, A) it has been carried by its backward growth 

 into the abdomen of the host, where it lies very close to the A'entral 



