The gregarinæ live in their perfect form chiefly in insects, crustaceans, and worms. Fig. 35 represents a gregarina very common in the libellulæ. The largest species inhabits the intestines of the lobster. My son has studied them very carefully, and published the results in the bulletins of the Academy of Belgium. [Illustration: Fig. 35.--_Stylorynchus oligacanthus_ from the larva of the Agrion.] Schneider has described a parasite which ought, no doubt, to be placed among the gregarinæ; it lives in the testicle, as well as in the salivary cells, of a planaria, the _Mesostomum Ehrenbergii_; Schneider represents the various phases of its development. In the autumn of 1871, nearly all the mesostomes perished through the presence of these parasitical organisms: in the following year they were rare. Some years ago, Kölliker discovered on the spongy bodies of molluscs, certain parasites, the nature of which appears still as enigmatical as on the first day of their discovery. The Würzburg professor gave them the name of _Dicyema_. We have had for a long time in our portfolio some observations upon them, and at the close of the chapter "On Parasites that undergo Transformations," we give a representation of a Dicyema which we found in abundance on the _Sepia officinalis_ off the coast of Belgium. CHAPTER VIII. 162 PARASITES THAT ARE FREE WHEN OLD. We are about to study in this chapter animals which seek for assistance from others while young, and are able to provide for themselves completely when they have grown old. We may compare the hosts which afford them shelter to _crèches_ which receive none except newborn infants. It is generally supposed that animals known under the name of parasites are such as require assistance from their neighbours during all the stages of their existence.[3] This is a mistake. There are very few among them which are not able to provide for themselves during some period of their development, and they then lead an independent life. We have mentioned a certain number of them in the preceding chapter, which only seek for external assistance when they are old; we bring together, on the contrary, in this chapter, those which require help at the commencement of their life, and live at large on their own industry when they have once made their entry into the world. There are even some among them which are richly endowed, 163 and one would never imagine that they would have recourse to strangers in order to bring up their progeny. All their young family is usually entrusted to the care of a nurse, who lives just long enough to bring them up; she gives them convenient shelter under her roof, and often bestows upon them the last drop of her blood.