1 78 MANUAL OF ZOOLOGY. 



much smaller than the females, which are usually furnished 

 with external ovisacs. 



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 Lerncea, Achtheres, Peniculus, &G. 



ORDER 1. 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 usually a pair of long, cylin- 

 drical ovisacs, depending from the extremity of the abdomen. 

 Some adhere by a suctorial mouth, or by cephalic processes 

 (Gephaluna') ; 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 (Onclmna) at- 

 tachment 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 

 cephalothorax 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 is free-swimming, and is provided with visual 

 organs and locomotive appendages. The two sexes are now 

 alike, and the conversion of the active embryo, or larva, into 

 the swollen and deformed adult must be regarded as an in- 

 stance of * retrograde metamorphosis.' 



SUB-CLASS II. CIRRIPEDIA. This sub-class includes, amongst 

 others, the common Acorn- shells, and the Barnacles or 

 Goose-mussels. All the Cirripedia are distinguished by the 

 fact, that, in the adult condition, they are permanently fixed to 

 some solid object by the anterior extremity of the greatly 

 metamorphosed head ; the first three cephalic segments being 

 much developed, and enclosing the rest of the body. The 

 larva is free and locomotive, and the subsequent attachment, 

 and conversion into the fixed adult, is effected by means of a 

 peculiar secretion, or cement, which is discharged through the 

 antennae of the larva, and is produced by a special cement- 

 gland, which is really a portion of the ovary. In the Cirripi'dia, 

 therefore, the head of the adult is permanently fixed to some 

 solid object, and the visceral cavity is protected by an articu- 

 lated calcareous shell, or by a coriaceous envelope. The pos- 



