420 



ARTUnOPODA. 



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 stall: which penetrates tlie bod}' of the host and a 

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



FlG. 4.31. — SacmJina cnrvini attached to Cfircinim itirenas, whose abdomen is extended. 

 )i>, shell opening; r, networlc of roots ranaifying the crab: .s, stalls : c, o. (7, anten- 

 nula, eye and anus of the crab. 



ner, penetrates the cephalotborax 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. 439) no great difference from that of ether cirripeds. These forms 

 are rare on the American coast. Sacciiliiia. Pdtoijaster* 



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

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



Suh Class Y. Malarostraai. 



The Malacostraca are sharply marked off from the other C'rtTS- 

 tacea by having a l:)ody which consists of twenty segments, of which 

 seven are al)dominal {Nelialia lias twenty-one, eight abdominal). 

 The excretory organs are represented by the anteiinal glands, and 

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

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

 setrnient. 



