WORMS. 



215 



end of the body, particularly the region of the oesophagus, is very much thickened ; in 

 Chmtosoma this thickening ends abruptly posteriorly. The body shows fine transverse 

 lines, and has hairs or bristles upon the back. The mouth is i 

 surrounded by three lips. The male has two hooks around ' 

 the genital aperture, which are very similar to the so-called 

 spiculaj of nematod worms. The most singular and charac- 

 teristic feature of CJuBtosoma, however, is a double row of 

 short rods on the ventral side, just in front of the anal open- 

 ing ; each rod has a knob at the free end. Nothing is known 

 of the life history of these interesting forms. Hitherto they 

 have been observed in Europe only, but it seems probable 

 that they will yet be discovered in America. 



The second type, the DKSMOscoLBCiDiB, is also imper- 

 fectly known. It presents some indications of segmentation 

 of the body, but whether it is really jointed like a true anne- 

 lid is very doubtful. We cannot do more than give a brief 

 description of a typical species, Desmoscolex minutus. The 

 animal has a head-like enlargement of the anterior end of 

 the body, and, behind that, a series of protuberant rings, pro- 

 ducing an appearance of segmentation. The head carries 

 two pairs, and each ring (save the eleventh and fifteenth) a 

 single pair of bristles, which Greef states are used as locomo- 

 tive' organs. The sexes are distinct, and may be recognized 

 by secondary external sexual characters. Three or four eggs 

 are laid at a time, and borne about by the female for some 

 while. The animal is marine. 



Class VIL — NEMERTEA. 



The nemertean worms are free-living animals, usually very 

 much elongated in shape, but especially characterized by an 

 enormously long slender proboscis, which when at rest, is 

 withdrawn into the body, but is thrown out upon slight 

 provocation. When one of these animals is captured, the 

 proboscis is often broken off and might readily be mistaken 

 for a second worm, for it is a long filament, which maintains 

 its autonomic contortions for a considerable period. The 

 Nemertea were long classed with the Turbellaria, but the 

 union with the latter was based upon gross misconceptions, 

 and we now know that they must stand as an independent 

 class, having more affinity with the true annelids than with 

 the plathelminths. 



We may take the well known genus Tetrastemma as 

 typical of the class, although most of the forms are very 

 much larger. The species represented in our illustration 

 is only two millimetres long, while some nemerteans ex- 

 ceed a foot in length. In Tetrastemma the opening of 

 the mouth, and also that of the canal, in which the re- 



111 



'/ 4 



Fio. 20i.— Tetrastemma obscura. 



