Parasitism and Social Reciprocity. 195 



which they were all differentiated, to which they are all subordinated, 

 and back into which they have now subsided. 



The history of parasitic and sedentary animals includes a youthful or 

 larval stage invariably more or less active, in which the animal possesses 

 some animal functions, at least locomotion and the necessary organs for 

 it, and a mature stage in which it only vegetates in the interest of repro- 

 duction, and in which a part and sometimes all the animal functions 

 are lost, The larval history of a parasite must indicate the race history 

 of the tribe to which he belongs, with greater or less fidelity, and so we 

 are bound to suppose that the ancestors of every parasite and sedentary 

 tribe were active and self-supporting and not at first parasites. We 

 have a few examples of partial and unfinished parasitism which confirm 

 this view. 



Semper mentions that some crustacean parasites, whose present mode 

 of life requires no organ of locomotion, still possess their swimming 



There are a great many parasites infesting various animals, especially 

 marine animals, both inside and outside ; and it often happens that the 

 endo parasites are of the same species as their friends on the outside, 

 the ecto parasites, but in almost every case there is a greater or less 

 degradation of structure in the one on the inside as compared with the 

 other. Among the many parasites infesting the Holothuriae (sea-slugs) 

 is the Eulima, a mollusk that lives on the skin of the sea-slug. It also 

 is found on the star fishes. It is in all particulars a univalve mollusk 

 except that it has lost its organ for gnawing and masticating its food, 

 an organ that is common to all univalves but now useless to the Eulima, 

 which lives on the slimy secretion of the host which it takes up by suc- 

 tion. 



Another Eulima inhabits the inside of the Holothurian, differing but 

 little from the outside one and destitute only of the masticating jaw or 

 rachis. There is another species of Eulima parasitic on the Holothu- 

 rian, fixed immovably on the outside, and having his jaw developed into 

 a trunk or proboscis which is inserted through the hide of the host and 

 sucks up all the nourishment required. The proboscis performs the 

 function of anchoring the parasite, an office usually performed by the 

 foot. But the foot is thus rendered useless and is now aborted. Loco- 

 motion not being indulged in, eyes are not necessary, and they, too, 

 are aborted. On the other hand, the much-imposed-upon sea-slug enter- 

 tains a number of endoparasites free, which roam about his intestines 

 and appropriate whatever they fancy. Some of these are Trematode 

 worms, others small Crustaceans belonging to the Copepods, Cyclops, 

 &c. They are probably comparatively new at this sort of life or are 

 subject to frequent disturbance because they are not mutilated as yet 



