On the Anatomy of Sacculina. 



307 



Peltogaster carcini, Rathke, and Pacliybdella Bathkei, Die- 

 sing. 



The Larva. PI. XIII. fig. 1. 



The larva, in the first stage, is oval, and presents no 

 marks of segmentation. Placed near the centre of the 

 anterior margin of the body is a yellow speck — the eye (a). 

 The ocellus is placed nearly in the centre of a dark-colonred 

 ring (6). Krohn, who has observed a structure similar to 

 this in the larva of a Balanide, regards it as the oesophageal 

 ring. The lateral margins of the body, on either side of the 

 ocellus, are prolonged into two horns (c) ; and in this re- 

 spect the young resembles the Cirripedian larva in its first 

 stage. It is provided with three pairs of natatory legs : the 

 first pair (d) are situated immediately posterior to the horns 

 of the carapace; they are uniramous, are provided at their 

 extremities with bristles, and appear to be composed of two 

 joints ; the second and third pairs (d' d") are larger than the 

 first, and are both biramous. The rami are furnished with 

 bristles. The under surface of the body is prolonged into 

 two spines (e e), which project beyond the posterior margin 

 of the carapace. Besides these terminal spines, I have obser- 

 ved through the transparent body, two other structures (ff), 

 which resemble very much the middle pair of spines described 

 by Darwin as occurring on the larva of Chthamalus stellatus. 

 The greater portion of the body is occupied by an oval mass 

 of nearly spherical globules (g). The various transforma- 

 tions of the larva remain yet to be determined. 



The Adult Animal. PL XIII, figs. 10, 11, 12, 15. 



According to the present state of our knowledge, this 

 parasite seems to be peculiar to the Decapod Crustacea. 

 All the specimens I have obtained have been attached over 

 the terminal portion of the intestinal canal (figs. 10 & 11 b') 

 of the crabs on which they were parasitic, the females of 

 which they appear to infest more than the males. This 

 latter circumstance seems to be owing to the large size of 

 the purse of the female, as compared with that of the male 

 crab, affording them a better protection and means of 

 support. 



The external sac (figs. 2, 4 a). — The external skin is a 



