ANNULOSA : CRUSTACEA. 2O9 



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

 velopmental history bear out this view, but cases are known 

 (in some Zernaa) in which the males do not undergo retrograde 

 metamorphosis, but remain permanently in the condition of 

 free Copepods. 



This division includes the single order Ichthyophthira, tine 

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

 class, comprising various parasites upon fishes belonging to the 

 genera Lerncea, 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 {Cephalund) ; others are attached by a suctorial disc, 

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

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

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

 the first pair of thoracic limbs. — (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 

 mouth, gullet, and intestine, terminating posteriorly in a distinct 

 anus. The nervous system consists of a double ventral cord. 



The embryo (fig. 63, a) is free-swimming, and is provided 

 with visual organs and locomotive appendages. The two 



Fig 63.— Ichthyophthira. a Free-swimming \xrvi. ai Achtheres ^rcanim, in its first 

 stage ;* Adult male of the same. Enlarged. (After Owen.) 



sexes are now alike, and the conversion of the active embryo, 

 or larva, into the swollen and deformed adult, must be regarded 

 as an instance of " retrograde metamorphosis." In Achtheres 



