426 ARTHROPOD A. 



Order III. Rhizocephala. 



These forms differ so much from the other cirripeds as to demand sepa- 

 rate mention. They are parasitic on the abdomens of various decapod 

 crabs and consist of a stalk which penetrates the body of the host and a 

 body which remains outside. The stalk, which branches in a root-like man- 



Fio. 431. Sacculina carcini attached to Carcinus mcenas, whose abdomen is extended, 

 ru, shell opening; r, network of roots ramifying the crab; s, stalk ; a, o, d, anten- 

 nula, eye and anus of the crab. 



ner, penetrates the cephalothorax and absorbs its juices. Since the stalk 

 furnishes the food, an alimentary canal is absent. The body lacks all ap- 

 pendages, is enclosed by a soft-skinned mantle, and is almost entirely 

 filled with the gonads. Since these forms lack, as adults, all arthropodan 

 features, their position is only settled by their development, which shows 

 (fig. 429) no great difference from that of other cirripeds. These forms 

 are rare on the American coast. Sacculina, Peltogaster* 



Two more orders, ABDOMINALIA and APODA, parasitic in the mantle 

 and shells of molluscs and other cirripeds, scarcely need mention. 



Sub Class V. Malacostraca. 



The Malacostraca are sharply marked off from the other Crus- 

 tacea by having a body which consists of twenty segments, of which 

 seven are abdominal (Nebalia has twenty-one, eight abdominal). 

 The excretory organs are represented by the antennal glands, and 

 shell glands are lacking except in some Isopoda. The male geni- 

 tal ducts open on the thirteenth, the female on the eleventh, 

 segment. 



