On the Anatomy 0/ Sacculina. 
307 
Peltogaster carcini, Eathke, and Pachyhdella 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-coloured 
ring (5). 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 (//), 
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. PI. 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 h') 
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 
