IV RHIZOCEPHALA 95 
Sub-Order 6. Rhizocephala.' 
These remarkable animals are Cirripedes which have taken 
to living parasitically on various kinds of Crustacea; the 
majority infest species of Decapoda, e.g. Peltogaster on Hermit- 
Crabs, Sacewlina on a number of Brachyura, Sy/on on Shrimps, 
Lernaeodiscus on Galathea; but one genus, Duplorbis, has been 
found in the marsupium of the Isopod Calathura brachiata from 
Greenland. Most of the species are solitary, but a few, eg. 
Peltogaster sulcatus, are social. In the adult state the body 
consists of two portions: a soft bag-like structure, external to the 
host, carrying the reproductive, nervous, and muscular organs, 
and attached to some part of the host’s abdomen by means of a 
chitinous ring; and a system of branching roots inside the host’s 
body, which spring from the ring of attachment and supply the 
external body with nourishment. 
The structure of the external bag-lhke portion is very simple, 
and varies only in details, chiefly of symmetry, in the different 
genera. In Peltogaster, 
which preserves’ the 
simplest symmetrical 
arrangement of the 
organs, a diagrammatic 
section through the 
long axis of the body 
(Fig. 66) shows that it 
consists of a muscular 
mantle (m) surround- 
ing a visceral mass, 
and enclosing a mantle- 
Fic. 66.—Nearly median longitudinal section (diagram- 
matic) of Peltogaster. gn, Brain; m, mantle ; mc, 
cavity (me) or brood- mantle-cavity ; mes, mesentery ; op, mantle-open- 
: ; ing; ov, ovary; ovd, oviduct ; ring, ring of attach- 
pouch, which stretches ment ; ¢, testis ; vd, vas deferens. 
everywhere between 
mantle and visceral mass, except along the surface by which 
the parasite is attached to its host, where a mesentery (mes) 
is formed. The ring of attachment is situated in the middle 
of this mesentery; the mantle-cavity, which is completely 
1 Y. Delage, Arch. Zool. Exp. (2), ii., 1884, p. 417; G- Smith, Pawna wu. Flora 
G. von Neapel, Monogr. 29, 1906. 
