ARTHROPODA 



lv, 



may be extended. Huxley's statement that " a barnacle may 

 be said to be a Crustacean fixed by its head, and kicking tin- 

 food into its mouth with its legs," is very fitting. 



The parasitic barnacles are very degenerated in structure 

 (Fig. 1 5 1 ), without appendages, and present no structural 

 evidence of belonging to the Cirripedia ; their classification is 

 based upon their development. The)- live attached to the ven- 

 tral side of crabs, into whose bodies they send numerous, finel) 

 blanching processes, which thus obtain nutriment for the para- 

 site. Externally there appears merely a sac, practically tilled 

 with the reproductive organs, which is attached to the host by a 

 short stalk. 



This is the only group of Crustacea in which hermaphro 

 ditism is the rule, this being to a large degree the necessary 

 consequence of the attached condition, though even here cross 

 fertilization often takes place. In a few cases dwarf or pygmy 

 males occur, which are not attached, but generally live within 

 the shells of hermaphroditic individuals, or in a few cases ol 

 female individuals; in the latter case, then, the sexes arc sepa 

 rate, but with great sexual dimorphism. In their development 

 all the Cirripedia pass through a free-swimming stage, and after 

 ecdysis pass into a stage like the adult Ostracoda, and hence 

 called the Cypris larva. This becomes attached at its anterior 

 end and is metamorphosed into the adult. The hard shell of 

 barnacles is not cast off by ecdysis, but only the delicate, chiti- 

 nous covering of the appendages and adjacent structures is shed 

 from time to time. 



CLASS II. MALACOSTRACA 



The Malacostraca (Gr. paXa/cos, soft, and oarpaKov, shell) 

 include the larger and more highly developed Crustacea, which 

 arc readily distinguished from the Entomostraca by the number 

 ot segments in the body. This number is constant, so that 

 there are five in the head, eight in the thorax, and seven in the 

 abdomen ; there is one exception to this rule, one subcl 

 having eight in the abdomen. Each segment except the lasl 

 bears a pair of appendages. The last segment is called the 

 telson. Some or all of the thoracic segments are fused with 

 the head so that we can distinguish a cephalothoracic region 



