Professor Semper found at the Philippine Islands, in the _Holothuria edulis_, another species of Entoconcha which appears to attach itself to the anal vent of this echinoderm. He gave it the name of _Entoconcha Mulleri_. We have in it a new example of the relations which certain parasites bear to their hosts, and which are the same in both hemispheres. The _Lichnophoræ_ are infusoria, allied to the _Vorticellæ_, whose form they assume; these are "mimic species," or mocking forms, of the Trichodinæ. One species, the _Lichnophora Auerbachii_ lives on the _Planaria tuberculata_; the other, the _L. Cohnii_, on the branchial membranes of the _Psyrmobranchus protensus_. The associations in the inferior ranks of animals have functions which are of the highest importance; some to maintain harmony and health in all that possess life, others to sow the seeds of death throughout whole regions. There are, in fact, associations in the ranks of the infinitely small creatures, which sometimes have the effect of purifying and rendering more healthful, sometimes of destroying. It is among these beings, invisible to the naked eye, that we must seek for the cause of some epidemic diseases. We have here an example of 160 what certain groups of animals are able to accomplish. The crustaceans everywhere perform the office of vultures to clear the waters from dead bodies, whether large or small, and they are in general sufficiently numerous to perform this police duty effectually. We may say that without their aid the waters along the coasts and at the mouth of rivers would grow speedily corrupt and unfit to support life. Thus it sometimes happens that when the number of these beings is insufficient, or the putrescible matter is in excess, we see the fish, the molluscs, and even the crustaceans, perish one after the other. [Illustration: Fig. 33.--_Gregarina_ of _Nemertes Gesseriensis_.] The last of the parasites of this category are known by the name of Gregarinæ. It appears that Goede was the first to make observations upon them. Léon Dufour gave them the name which they still bear. They have a very simple organization, and are formed only of a cell which contains a nucleus: they live in the intestines of many invertebrate animals, especially in the articulata. Let us imagine a body, long, more or less transparent, with a smooth surface very like a spindle, which glides about in the intestines, in the midst of the liquid matter which it contains, without our being able to ascertain the 161 mechanism by which it moves (Fig. 33.) While young they are encysted, and bear the name of _Psorospermiæ_. Fig. 34 represents one of these sacs of Psorospermiæ from a cephalopod. [Illustration: Fig. 34.--Sac with Psorospermiæ from the _Sepia officinalis_.]