374 ARTHROPODA 



Order II. Balanidae. 



Sessile cirripeds with calcareous shell formed of carina, rostrum, and la'er- 

 alia; scuta and terga forming the valves (fig. 390). Balanus* Coronula, attached 

 to whales. 



Order III. Rhizocephala. 



These differ greatly from other cirripeds. They are parasitic on the abdo- 

 mens of decapod crabs and consist of a stalk which penetrates the body of the 

 host and a body which remains outside (fig. 394). The stalk branches in a 

 root-like manner, penetrates the cephalothorax and absorbs its juices. Since 

 the stalk furnishes the food, an alimentary canal is absent. The body lacks 

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



"" "d 



FIG. 394. Sacculina carcini attached to Carcinus mcenas, whose abdomen (d~) is ex- 

 tended. /, sex opening of Sacculina; r, network of roots ramifying the crab; 5, stalk. 



filled with the gonads. Since the adult parasites lack all arthropodan features, 

 their position is only settled by their development (fig. 393), which is like 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 Crustacea 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 the larvae and some Isopoda. The male genital ducts open on 

 the thirteenth, the female on the eleventh, segment. 



