Sab-Class 1. Entomostraca. Order 7. Cirripedia. 



207 



peduncle ; the tlu-eads of the anterior region twine round the internal organs 

 of the host, and absorb food by endosmosis ; they are comparable, both in general 

 appearance and in function, with the roots of a plant. The saccular part is 

 covered by a soft mantle; the mantle cavity, in which the eggs are retained, 

 communicates with the exterior only by a small aperture. Digestive tract and 

 limbs ai-e altogether wanting. The Rhizocephala undergo a, metamorphosis 



Fig. 169. A Ehizo- 

 cephalon (SaccuUna), 

 3, on the ventral side of 

 the abdoiren of a Shore 

 Crab ; the Crab is seen 

 from below, with the 

 abdomen artificially 

 stretched out. — Orig. 



whose first stages are like those of a normal Cirriped (nauplius, cypris) ; after 

 attachment to the host, however, the animal undergoes a modification, which 

 results in. the above- described abnormal structiu-e. They are parasitic upon 

 Decapoda ; one species {Sacculina carcini), for instance, is vei-y commonly found 

 on the ventral side of the abdomen of the Common Crab {Carcinus moenas) of 

 European coasts ; another {Peltogaster paguri) on the abdomen o the Hermit- 

 crab (the " roots " in both cases permeate the whole body of the host, whose 

 genitalia do not ripen). 



Sub-Class 



Malacostraca. 



In contrast to tlie Entomostraca^ where the number of segments 

 and of limbs varies within very wide limits, there is a typical 

 number in the Malacostraca, which is never exceeded, but which 

 may be reduced in some forms by the loss of some segments or 

 pairs of appendages. 



The body is divided into three regions: the lie ad, the 

 thorax, consisting of eight segments, and the abdomen, of 

 seven. From the head arises in most orders a carapace, 

 which never covers more than the thorax (often not the whole 

 of it), and leaves the thoracic limbs and abdomen uncovered (c/., 

 the Daphnidae, the Phyllopoda, and others) ; the carapace is 

 always confluent with a certain part of the dorsal side of the thorax, 

 whilst its sides are free ; its outer surface' is covered with a hard 

 cuticle, which often attains a considerable thickness. The head bears, 

 further, a pair of large compound lateral eyes, which are usually 

 stalked and movable, whilst the nauplius-eye is generally absent from 



