236 MANUAL OF ZOOLOGY SECT. 



fixed in the adult condition as external parasites, mainly 

 of fishes. Many of those parasitic Entomostraca undergo 

 a degradation of structure, a retrograde metamorphosis, as it 

 is termed. Comparatively highly organised in their free- 

 swimming larval stages, these lose when they attain the adult 

 parasitic condition some, if not all, of their characteristic 

 crustacean features, and may lose all trace of segmentation 

 and of jointed appendages. Also characterised by degrada- 

 tion of structure, though in a less degree than some of the 

 parasitic forms, are the barnacles (Fig. 136) and acorn- 

 shells (Cirripedes), which are not parasitic, but are perma- 

 nently fixed in the adult condition to a rock or a beam of 

 timber or other submerged object. In the larval condition 

 these are free-swimming, distinctly segmented, and provided 

 with a number of jointed appendages ; in the adult state they 

 become fixed, lose their segmentation, though retaining some 

 of their jointed appendages, and become enclosed in a fold 

 of the integument in which are developed a series of cal- 

 careous plates. The attachment of the cirripede is by the 

 head; while the posterior portion of the body is free, and is 

 capable of being thrust out with a series of six pairs of many- 

 jointed appendages or cirri, borne on the thorax through a 

 slit in the enclosing shell. In the barnacles the head-region 

 is drawn out into a stalk (A, /) ; in the acorn-shells the stalk 

 is absent. 



2. ONYCHOPHORA 



The class Onychophora comprises only the aberrant genus Peripatus, 

 which is interesting owing to certain primitive features which it presents 

 features which afford some reason for regarding it as intermediate 

 between such forms as the Annulata on the one hand, and the higher 

 Arthropoda on the other. 



Peripatus (Fig. 137) is a caterpillar-like animal of approximately 

 cylindrical form, and not divided into segments; it has a fairly well- 



