SECT. X 



PHYLUM ARTHROPODA 



235 



phosis, as it is termed. Comparatively highly organised in 

 their free-swimming larval stages these lose some, if not all, 

 of their characteristic Crustacean features by the time they 

 attain the adult parasitic condition, when all trace of segmenta- 

 tion and of jointed appendages may have disappeared. Also 



FIG. 127. Lepas anatifera. A, the entire animal, a. antennule ; c. carina ; 

 cd. cement gland ; /. digestive gland ; m adductor muscle ; od. oviduct ; <n>. 

 ovary ; p. penis ; s, scutum ; /. tergum and testis ; I'd. vas deferens. B, anatomy. 

 c. scutum ; /. peduncle ; s. carina ; /. tergum. (From Lang's Comparative 

 Anafomy, after Darwin and Claus.) 



characterised by degradation of structure, though in a less 

 degree than some of the parasitic forms, are the Barnacles 

 (Fig. 127) and Acorn-shells (Cirripedes), which are not 

 parasitic, but are permanently fixed in the adult condition 

 to a rock or a beam of timber or other submerged object. In 



