In a later part of this work, we shall mention worms which live in freedom in damp earth, and whose direct progeny, entirely composed of females and hermaphrodites, can only exist as parasites. These worms do not resemble their mother but their grandmother, and if their descent had not been traced, they would doubtless have been taken for species entirely distinct from each other. Thus it is not always the whole family which is modified; the male often preserves all the 140 attributes of his sex and of his youth, while the female changes entirely her appearance and her mode of motion, especially at the approach of the period when the interest of the species prevails over that of the individual. We can nowhere find more graceful and regular forms during the whole of their early youth than those of many of these parasites; we can never see more ungraceful, we might almost say more comical, attitudes than those of the greater part of these creatures when full grown. One might take them for some misshapen excrescence, or some scrap of wasted flesh on the body of their host. A certain number of insects are found which lead this singular kind of life, but this is more especially the case among the crustaceans, particularly the copepod crustaceans. Among all these we find the most absurd recurrent forms; in fact these animals instead of carrying on their evolution, like the caterpillar which becomes a butterfly, retrograde rather than advance, and acquire an appearance and character which prevent us from recognizing their origin. Many of these are at present known, whose graceful form is so completely changed, that without referring to the study of their embryo state, one could not tell to what class they belong. Nothing remains of their organs except the sexual apparatus and a shapeless skin. These curious parasites live also on the surface of bodies, and sometimes in the cavity of the mouth; but in fishes they are most frequently found in the branchial membranes. They look like natural setons, and it is not impossible that they sometimes fulfil the same functions. We will first examine some insects, then certain isopode crustaceans, 141 an order to which the Cloportidæ (wood-lice) belong, many of which require uninterrupted assistance; then we will turn to the Lernæans, which surpass all the rest in their many and bizarre transformations. [Illustration: Fig. 26.--Male Chigoe.] [Illustration: Fig. 27.--Head of Chigoe.] [Illustration: Fig. 28.--Female Chigoe.]