The members of this order (Lernaea, Achtheres, Peniculus, Caligus, Argulus, etc.) 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 (fig. 139). 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 (Brachiuna); whilst in others (Onchund) attachment is effected by hooks at the free extremities of the first pair of thoracic limbs (Owen).
The males (fig. 140, b) are usually not attached, but adhere to the females, 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. There are no differentiated breathing-organs, and respiration is effected by the surface of the body.
The embryo (fig. 140, a) 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 instance of "retrograde metamorphosis." In Achtheres percaum (fig. 140), as generally in the order, the primitive form of the young is a "Nauplius;" * but a wholly different larva, resembling the Cyclops in shape, but with fewer limbs and somites, is prepared within the Nauplius-skin, and is liberated by the rupture of the same.
Fig. 139. - Female of Achtheres Carpenteri, magnified. The line placed alongside of the figure shows the real size. a Abdomen ; d Disc of attachment developed upon the last pair of thoracic limbs; o Ovisac.
As regards their affinities, the Ichthyoph-thira are closely allied to the Copepoda, and may, indeed, be regarded as parasitic Cope-pods, having the mouth modified so as to form a suctorial tube or beak, resulting from the elongation of the labrum and labium. Within this are 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 developmental history bear out this view, but cases are known (in some Lernaeae) in which the males do not undergo retrograde metamorphosis, but remain permanently in the condition of free Copepods.