212 DESCRIPTIONS OF PREPARATIONS. 



giant tubular fibres. They are found in all Oligochaeta except in Aeolosoma, Phrae- 

 orycteS) and Branchiobdella^ and the number given is usually that found in the Earth- 

 worm. They occur also in many Polychaeta, where their number varies from one 

 as in Eunice^ to six as in Glycera. These fibres do not extend into the oesophageal 

 commissures, and they taper anteriorly and posteriorly. Each fibre is composed of 

 a doubly contoured sheath with clear contents. No connection between the giant 

 fibres and nerve fibres has ever been demonstrated. On the contrary the giant 

 fibres are separated from the nerve-cord by the inner neurilemma, and they are im- 

 bedded in a connective tissue sheath containing reticulate cells. They appear to 

 have a purely supporting function, and the apparatus is hence termed ' Neurochord ' 

 by Vejdovsky (op. cit. pp. 86-87), who compares it physiologically with the noto- 

 chord of Chordata. 



Nervous system. Claparede, op. cit. ante, p. 585. Leydig, Vom Bau des 

 Thierrischen Korpers, i. Tubingen, 1864, pp. 139, 168. Id. Tafeln zur Vergleich. 

 Anatomic, Tubingen, 1864, Taf. i. fig. 5; Taf. iii. fig. 8; Taf. iv. figs. 7 and 8 ; 

 Taf. v. figs, i and 2. Vignal, A. Z. Expt. (2), i. 1883, p. 373. Of Oligochaeta in 

 general. Vejdovsky, op? cit. pp. 79-96 ; and on Neurochord^ p. 87. 



Organs of special sense in Oligochaeta. Vejdovsky, op. cit. pp. 96-100. 



42. MEDICINAL LEECH (Hirudo medicinalis), 



Suspended to show the external form of the body and the coloured bands which differentiate the 

 variety H. medicinalis from the variety //. officinalis. 



THE animal is suspended by the anterior extremity which is formed 

 by the funnel-shaped buccal cavity or anterior sucker which leads to the 

 mouth, and is not separated from the rest of the body by a constriction in 

 Leeches with jaws (Gnathobdellidae) as it is in Leeches with a protrusible 

 proboscis (Rhynchobdellidae). The body itself has a flat or slightly concave 

 ventral surface and a convex dorsal surface : and it is terminated by a disc- 

 like solid posterior sucker which is formed by the fusion of posterior em- 

 bryonic somites, according to Leuckart seven in number. 



The body is annulated, and, according to Whitman, H. medicinalis has 

 in all one hundred and two rings, representing twenty-six somites. Conse- 

 quently the annuli do not represent somites : they are, on the contrary, due 

 to a secondary and imperfect division of them. It has been pointed out 

 by Whitman that certain of the annuli or rings bear what he terms seg- 

 mental papillae. These organs resemble in histological structure the eyes, 

 but reduced in size and deprived of pigment, and they may be regarded 

 as the metameric or serial homologues of those organs. In Hirudo and 

 some other Leeches there are normally fourteen segmental papillae, eight 

 on the dorsal and six on the ventral aspects of the annuli upon which they 

 occur. The eight dorsal papillae are arranged as a median pair, with 

 three organs to either side of it, an inner, outer, and marginal organ. The 



