ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1005 



organ ; it lias the form of an ovoid mass, and is clear and refractive ; 

 it is bounded at the sides by a double layer of ganglionic cells, and a 

 few are to be fonnd in its interior. The whole apparatus is very 

 mobile, and the animal seems to be incessantly testing by means of the 

 papilla3 which terminate it. It is best developed in young specimens 

 which have just escaped. 



The nerves are everywhere surrounded by an endothelial sheath, 

 the cells of which are continuous with those of the reticulum; 

 between the nerve and its sheath there is a cavity which is continuous 

 with a system of lacunas that occupy the whole layer of zoochlorellae. 

 Each of these algae is contained in a free cavity, the spaces between 

 which are formed by the lacunae. 



Phoenicurus.* — In studying Tethys leporina Prof, de Lacaze- 

 Duthiers found a number of the incompletely known parasite which has 

 been named Phoenicurus. After a short description of the external 

 form of the animal, and an account of the incessant changes to which 

 it subjects it, he describes the nervous system as consisting of two 

 ganglia united by a long transverse commissure; each gives off two 

 primary nerves, one of which goes to the region of the mouth, and the 

 other (lower) to the tail. The ganglia are small, and contain a small 

 number of large nerve-cells. There are a number of secondary 

 nerves which arise from the ganglia and pass to all parts of the body ; 

 they are ordinarily very delicate, very long, and generally wavy, a 

 condition which is to be correlated with the changes in dimension 

 which are undergone by the body. The two superior primary nerves 

 give off, on their course, delicate branches which pass into the sub- 

 cutaneous tissue of the buccal fossa ; as they get to some distance 

 from their centre they are seen to have ganglionic swellings, which 

 vary considerably in size, and are composed of one, two, or three 

 cells which are elongated along their great axis, which lies parallel 

 to the direction taken by the nerves. It is very remarkable that no 

 two individuals are entirely alike in the composition of their nervous 

 centres ; sometimes there is one median ganglion, sometimes a kind 

 of chain of three or four, and in one case there were as many as seven 

 ganglia, united by a plexus. It is not rare to find only one buccal 

 nerve, which is then of large size. In fact, the position of the 

 nervous system is constant, but its forms vary infinitely. 



Phoenicurus is acojlomate, and a fibrillar cellular tissue containing 

 a number of nuclei takes the place of the body-cavity. Under the 

 skin and a layer of connective tissue there are longitudinal muscular 

 bands, which form a dorsal and an abdominal layer, and extend from 

 one end of the body to the other. In addition there are transversely 

 set external bands, which form a complete network. On either side 

 there are aggregations of muscular fibres which run pei-pendicularly 

 to the sui-face, and aid in limiting the central space. 



In this space lies the digestive tube, the central nervous system, 

 and a si>ecial gland. The tube commences with an orifice placed in 

 tlic buccal fossa, and extends to the tail ; its arrangement is dendro- 



* Comptes Kciiclus, oi. (1885) ])p. SO-.l. 



