Most of the inferior crustaceans require assistance from others: some might be correctly arranged as messmates, but the whole category of the Lerneans is so low in development that Cuvier placed them by 148 the side of the helminths. These creatures possess as soon as they are born, all the attributes of their class, and wear the dress of free crustaceans; as they approach mature age, they choose a neighbour, instal themselves as conveniently as possible in one of his organs, and get rid of all their apparatus for fishing and hunting. The sexes are usually separated, and as the female is specially devoted to the cares of her progeny, she is the first to give up her liberty. Sometimes the male, not content with leaving to her all the trouble of providing for the family, demands from her his daily food, and establishes himself like a spermatophore on her sexual organs. It is only right to say that in this case, the male sex is far from being the stronger, for he is often less than the tenth or even the hundredth part of the size of the female. At last we see the female lose her claws and her swimming apparatus, while the male keeps his carapace with all his appendages of the senses and of locomotion. The difference between the two sexes is so great in some species, that it would be impossible to imagine that a brother and sister could assume such dissimilar forms, unless we had watched them from the time when they first issued from the egg. The female is a kind of puffed-out worm, and the male resembles an atrophied acarus. This explains why the female was known so long before the male, whose office is only that of reproduction. Nordmann, during his residence at Odessa, was the first to begin these researches, which have been continued by Messrs. Metzger and Claus. It is known that the Lerneans attach themselves to their hosts by indissoluble bonds, only becoming parasites after they have passed 149 their youth in complete independence, and have all possessed the graceful forms so characteristic of the _Nauplius_ and the _Zoë_. When they first leave the egg, they swim about in freedom, but at length some day the female, thinking of a family, looks out for a neighbour that can give her the assistance she requires, fixes herself on his skin, and rapidly develops till she is two or three hundred times as large as the male; her head, her body, and her stomach become of a monstrous size, a part of her head is often anchylosed in the bones of her host; the lernean remains suspended as a sort of festoon, to which are afterwards joined two ovisacs filled with eggs. Fig. 30 is a lernean of a fresh-water fish, represented at different periods of its existence. [Illustration: Fig. 30.--Tracheliastes of the Cyprinæ. 1, larva, as it leaves the egg; 2, larva, more advanced; 3, adult female, attaching itself before and behind to two ovisacs (Nordmann).]