150 M. A. Giard on the Genus Entoniscus. 



tlie ventral side, lateral vessels which send forth branches 

 to the fringed plates of the abdomen. 



These fringed laminae may be regarded as true branchiae. 

 Moreover they occupy the position of the branchial laminae of 

 the normal Isopods. Their excessive development in the 

 Entonisd is easily explained in the following manner. 



We have stated that the Entoniscus in the body of its host 

 is completely surrounded by a fine membrane. This mem- 

 brane does not belong to the parasite ; it is the continuation 

 of the membrane which clothes the viscera of the crab and 

 separates them from the branchial cavity. This membrane is 

 gradually drawn back by the growth of the Entoniscus, which 

 is thus enclosed in a sort of pouch formed by invagination. 

 From this it results that the Entoniscus, as Fritz Mtiller 

 justly points out, is an external parasite, although it appears 

 to be in relation with the most internal viscera of its host. 



That the Bopyridae need well aerated water constantly 

 renewed, appears clearly from the position which they take 

 up in the various animals to which we find them attached. 

 The typical Bopyri lodge themselves in the branchial cavity 

 of the Macrura and Anomura, where they draw from their 

 host a revivified blood, and themselves find constant fresh 

 supplies of water. Therefore their respiratory apparatus is, 

 in general, but slightly developed. The Phryxi attach them- 

 selves to the abdomen of Paguri at the spot where the ova 

 are collected in the females of those animals — that is to say, 

 at the point where the movements of the infested animal 

 also allow of a ready renewal of the water. Nevertheless, 

 as this renewal is less perfect than in the preceding case, 

 the abdominal laminae are already much better developed 

 than in the Bopyri properly so called. 



In the Entonisci the position of the animal, in a deep in- 

 vagination of the inner wall of the branchial cavity of the 

 crabs, renders respiration much more difficult. Hence the 

 respiratory lamellae have attained a much more considerable 

 development, and their undulated and crisped surface converts 

 them into regular sponges constantly impregnated with liquid. 

 Their movement of contraction, however, enables them to 

 drive off this liquid, and to draw in fresh supplies when the 

 necessity for so doing is felt. 



In E. porcellance, in which the abdominal feet have re- 

 tained an ancestral form, it is the appendages of the thorax 

 that have been modified and converted into undulated 

 lamellae. 



It is clear, moreover, that these various peculiarities are 

 serviceable not only to the adult Bopyridae, but also to their 



