The _Gregarinæ_ are microscopic beings, with an extremely simple organization, the nature and the genealogy of which have only lately been known. They live at first encysted by thousands together, under the name of _Psorospermiæ_; they are afterwards hatched in the form of _Amoebæ_, and then transformed into Gregarinæ. They migrate from one animal to another, or from one organ to another, to settle in the intestine, where they assume their adult form. In this state they are monocellular, and do not at any time possess organs which resemble the sexual organs of other classes. The disease of silk worms, known by the name of "pebrine," has been attributed to the development of psorospermiæ. We give the representation (Fig. 74) of gregarinæ which we have found abundantly on the Nemertes; and (Fig. 75) a peculiar species which lives in the larva of an agrion. [Illustration: Fig. 74.--Gregarinæ of _Nemertes Gesseriensis_.] [Illustration: Fig. 75.--_Stylorhynchus oligacanthus_, from the larva of the Agrion.] We also give a sketch (Fig. 76) of some very remarkable parasites, 254 whose affinities are still problematical, and which only inhabit spongy bodies, such as the kidneys of cephalopods. The name of _Dicyema_ has been given to them. [Illustration: Fig. 76.--_Dicyema Krohnii_, from Sepia officinalis.] Prof. Ray Lankester has quite recently made some very interesting observations, at Naples, on these problematical beings; and my son has just devoted a part of his vacation, with two of his pupils, to elucidate the points of their organization and development, which are still obscure. He went to reside at Villefranche, near Nice, in order to obtain fresh cephalopods every day. His observations have led him to a result quite different from that which I expected. CHAPTER X. 255 PARASITES DURING THEIR WHOLE LIFE. In this chapter we bring together true parasites, which may be called complete; they pass every part of their life under the care of a neighbour, and require an asylum the more urgently, since they cannot exist without it. They absolutely need both food and lodging. Not long ago, all parasites were supposed to be dependant during their whole life, and to be incapable of living outside the body of another animal. We have before proved that this opinion was erroneous. We find in this category a great number of parasites which may be separated and placed in the first group, including all such as pass all the phases of their life on the same animal, without changing their costume, and many of which never leave the fur, the feathers, or the scales, among which they are born.