ANNULOSA : CRUSTACEA. 227 



than the females, which are usually furnished with external 

 ovisacs. The Epizoa are closely allied to the Copepoda, and 

 may, indeed, be regarded as parasitic Copepods, having the 

 mouth modified so as to form a suctorial tube or beak, result- 

 ing from the elongation of the labrum and labium. Within 

 this are almost always two stylets or lancet-shaped mandibles, 

 used in piercing. The feet are often deformed by age, or 

 wanting, but are primitively natatory. Not only does their de- 

 velopmental history bear out this view, but cases are known 

 (in some Lerncza) in which the males do not undergo retrogade 

 metamorphosis, but remain permanently in the condition of 

 free Copepods. 



This division includes the single order Ichthyophthira, the 

 characters of which are therefore the same as those of the sub- 

 class, comprising various parasites upon fishes belonging to the 

 genera LeriKza, Achtheres, Peniculus, &c. 



ORDER I. ICHTHYOPHTHIRA. The members of this order 

 are attached in the adult condition to the skin, eyes, or gills of 

 fishes, and when mature possess an elongated body, having a 

 more or less distinct head, and in the females usually a pair of 

 long, cylindrical ovisacs, depending from the extremity of the 

 abdomen. Some adhere by a suctorial mouth, or by cephalic 

 processes (Cephaluna) ; others are attached by a suctorial disc, 

 developed at the extremities of the last pair of thoracic limbs, 

 which are united together (Brachiuncf) ; whilst in others ( Onchu- 

 na) attachment is effected by hooks at the free extremities of 

 the first pair of thoracic limbs (Owen). 



Fig. 84. Ichthyophthira. a Free-swimming larva of Achtkeres percarum. in its first 

 stage , b Adult male of the same. Enlarged. (After Owen.) 



The males are usually not attached, but adhere to the fe- 

 males, of which, from their much smaller size, they appear to 

 be mere parasites. The chief anatomical peculiarities of the 

 female are the following : The head is provided usually with a 

 pair of jointed antennae, and the body is divided into a cepha- 

 lothorax and abdomen. The alimentary canal consists of a 



