292 NEMATHELMINTHES. 



Chaetosomatidae. * The anterior end of the body is swollen into a head, with 

 a semicircle of moveable hooks. The body is covered with fine hair. There is 

 a terminal mouth, an oesophagus, intestine and ventral anus. Two rows of 

 knobbed processes on the ventral surface of hind end of body. Dioecious. 

 Testis single, vas deferens opens with the anus, and is provided with two 

 spicules. Two ovaries, single vagina, opening ventrally about middle of body. 

 Minute. Marine. Chaetosoma Clap., Rhabdogaster Metschnikoff. 



Desmoscolecidae.f Minute, vermiform marine animals. Body marked with 

 transverse ridges, which carry bristles. Mouth anterior and terminal, anus 

 dorsal. Dioecious. Testis and ovary as impaired tubes. Male with two 

 spicula, vas deferens opens with the anusf ovary opens on ventral surface. 

 They move by looping. Nervous system unknown. Desmoscolex Clap. 

 (Fig. 237), Trichoderma Greef. 



Class II. NEMATOMORPHA. + 



Elongated, unsegmented round worms without lateral lines, witli 

 ventral nerve-cord and circumoesophageal ring. 



The position of these worms in the system is obscure. The name 

 chosen for the order expresses the fact that in general form of body 

 they resemble the Nematoda. Their anatomy, however, is very 

 different from that of the latter group. The body is elongated and 

 filiform, but is without oral papillae and lateral lines. There is a 

 marked cuticle, beneath which is a cellular epidermis. The body- 

 muscles are in one layer of longitudinal fibres. The nervous system 

 consists of three cords closely approximated in the middle ventral 

 line, within the muscular layer. This ventral cord divides in front 

 into the two cords which embrace, and unite with one another dorsal 

 to, the oesophagus. The mouth (which is anterior) and oesophagus 

 are occluded in the adult state, and the anus is at the hind end of 

 the body. 



Between the body-wall and the gut there lie polygonal cells, 

 which in the adult seem to differentiate into various organs, e.g. 

 genital glands, genital ducts ; and a space, which may or may not be 

 a coelomic space, appears among them. 



* E. Metschnikoff, Z. f. w. Z., 17, 1867. Panceri, Atti Accad. delle Scienze, 

 Naples 7, 1878, p. 7. 



f Greef. Unters. lib. einige merkwiirdige Thiergruppen der Arthropoden u, 

 Wurmtypus, Arch. Naturg. 35 (1) 1869. Panceri, Op. cit. 



I F. Vejdovsky, " Zur Morphologic der Gordiiden," Z. /. w. Z., 43, 1886, 

 p. 369, and Ibid., 46, 1888, p. 188. v. Linstow, "Ueb. d. Entwick. und 

 d. Anat. v. Gordius," Arch. f. Mic. Anat., 34, 1889, p. 248. Id., " Weitere 

 Beobachtungen an Gordius," Arch. f. Mic. Anat., 37, 1891, p. 239. Id., "Uber 

 d. Entwick. v. Gordius," Cenlralbl. Bctct. ParasitJc., 11, p. 475. 0. Biirger, 

 "Zur Kenntniss v. Nectonema agile Verr.," Zool. Jahrb. Morph. Abth., 4, 1891, 

 p. 631. H. B. Ward, "On Nectonema agile," Bull Mus. Harvard Coll., 23, 

 1892, p. 135. 



