856 AUSTRALIAN HIRUDINEA, II., 



the posterior region, as tessellated. The number of annuli in 

 an unabbreviated somite is five (5) as in Herpobdella and many 

 other Arhynchobdellids. 



There are present no segmental papillae of any kind, and, as 

 no nephridiopores could be detected in the examination of the 

 entire animal nor any external signs of the metameric constitu- 

 tion, I have had to rely on sections. The separation of the male 

 and female genital apertures by five annuli inclined me, before 

 sectioning, to regard the somite as being pentannulate. The 

 absence of any metameric papillae, etc., which might serve to 

 distinguish any one annulus from another, is a characteristic of 

 the Herpobdellidce in general. 



Although one might be inclined, and reasonably so, to regard 

 the pentannulate nature of the somite as being of generic value, 

 still such is not invariable in some forms. For instance, the 

 three Japanese species of Orobdella have, in an unabbreviated 

 somite, four, six, and eight annuli respectively; and yet the 

 internal structures of the three forms show such strong mutual 

 affinities, that this variation is rightly counted as of mere specific 

 importance. We find a similar variation in somitic constitution 

 in species of Trocheta; and, in the form under description, 

 secondary ambulation reaches in places an important development. 

 In making out the somites, owing to the absence of papillae 

 and the difficulty in detecting nephridiopores in an examination 

 of the entire animal, I have had to rely on sections cut 

 longitudinally. 



I have adopted the views put forth by Castle, that the ganglia 

 of the ventral nerve-chain lie, not in the first annulus of a somite 

 as stated by Whitman, but in the middle annulus of an unabbre- 

 viated segment. In addition to the proof given to his statements 

 by Castle, this position of the ganglion, when we take into con- 

 sideration the method of somitic extension and its object, if we 

 are to consider the Hirudinea as being derived from a stock 

 allied to the Oligochaeta, fits in very well as a natural and not : 

 unexpected conclusion, the middle ring representing the stable 

 component of a somite. 



