CRUSTACEA OUTER ORGANISATION 



305 



all and nearly related to the Cirripedes. In the course of their development they 

 pass through stages similar to those of the Cirripedes, free-swimming and provided 

 with Crustacean limbs. 



In many Cirripedes there are by the side of the ordinary hermaphrodite 

 individuals, complementary dwarf males differently formed, which will be referred 



to later. 



II. Leptostraca. 



In the segmentation of the trunk, the Leptostraca (Fig. 196) take an inter- 

 mediate place between Entomostraca (especially Phyllopoda) and Malacostraca. As 

 in the latter, the thorax which follows the head consists of eight limb - bearing 

 segments, here very short but distinct. After the thorax comes the well-developed 



-mb 



FIG. 208. Sacculina carcini in situ on the host (after a somewhat diagrammatic original drawing 

 by Professor Delage, Paris), br, Branchial region ; I, hepatic region ; d, intestinal region of the host 

 (Carcinus) ; ks, body ; p, peduncle of the Sacculina ; mb, basilar membrane out of which the roots of 

 the parasite proceed. 



powerful abdomen consisting of 8 segments, i.e. of one more segment than the 

 typical Malacostracan abdomen of 7 segments. The abdomen is further followed 

 by the 2 so-called furcal processes. Only the 6 anterior abdominal segments carry 

 limbs. The 6 limb-bearing abdominal segments must correspond with the 6 anterior 

 abdominal segments of the Malacostraca. On the dorsal side of the posterior head 

 region an integumental fold arises, which in the shape of a delicate laterally 

 compressed bivalve shell covers the thorax and the 4 anterior abdominal segments, 

 but does not fuse with them. This shell also, like the corresponding shells of many 

 JSntomostraca, covers a large part of the limbs (Fig. 196). The head carries on each 

 side a stalked compound (facet) eye. 



III. Malacostraca. 



There are two peculiarities specially to be noticed in the segmentation of the body 

 of this large division. 



VOL. I X 



