a new Group of Parasitic Crustacea, 49 



hind the last pair of feet, a slight constriction of the body is 

 sometimes to be seen. 



The larva of the Sacculina differs in having a much larger 

 carapace extending far beyond the frontal horns and terminal 

 points, in the want of the eye, in the more oval form of the body, 

 and in its straight frontal horns, directed obliquely forwards. 

 I also found in it the bristles near the anterior margin, which 

 are wanting in Lernceodiscus, and behind the last pair of feet 

 on each side of the intestine an accumulation of brown opake 

 granules (urine?), of which also I can find nothing in LernceO' 

 discus. 



From the descriptions just given, the following peculiarities 

 may be indicated as characteristic of the Rhizocephala, which 

 hold an intermediate position between the Siphonostoma and 

 Cirripedia : — 



Crustacea Rhizocephala. 



Larva with three pairs of natatory feet, of which the two 

 posterior are biramose, with lateral frontal horns, two points at 

 the extremity of the abdomen, and a membranous carapace. 

 Mature animal soft-skinned, inarticulate, destitute of eyes, 

 antennae, feet, and mouth (?). Head immersed in the host, 

 hardened at the base into a chitinous coronet, taking up nourish- 

 ment by means of rootlike csecal processes. Hermaphrodites 

 with motile spermatozoids (?), without ovisacs (like the Cirri- 

 pedia), and with a wide brood-chamber open posteriorly. 



GENERA. 



1. Peltoff aster, Rathke*. 



2. Sacculina. Body unsymmetrical, sausage-shaped; head in 



the middle of the ventral surface. Larva eyeless, with two 

 frontal bristles. 



3. Lernceodiscus. Body symmetrical, disciform ; head at the 



anterior margin of the disk. Larva with an eye, without 

 frontal bristles. 



EXPLANATION OF PLATE H. 



Fig. 1. LerncBodiscus attached to the tail of the Porcellana; slightly 

 magnified. 



Fig. 2. A smaller specimen from the ventral surface, magnified 15 diame- 

 ters : a, aperture of the brood-chamber ; b, ovary ; c, testes (?) j 



* After the lapse of more than fifteen years, the details of Rathke*s 

 descriptions have escaped me too thoroughly to permit my characterizing 

 this genus, or even deciding whether Sacculina should not be united 

 with it. 



Ann. ^ Mag, N, Hist, Ser. 3. Vol. x. 4 



