L'.-,S "TERRA NOVA" EXPEDITION. 



Anterior sucker small, circular and narrower than half the greatest width of the 

 body. The mouth-opening perforates the upper surface of its interior cup at a point 

 situated about midway between the centre and the somewhat thickened rim. 



Posterior sucker large and powerful, slightly oval, centrally attached and broader 

 than the body at its widest part. 



There are no eyes and no lateral pulsating vesicles. 



A well-marked clitellum is present, its terminal rings being separated from the 

 anuuli contiguous to them by exceptionally deep grooves. 



There are normally 14 rings between the anterior sucker and the clitellum, but in 

 one individual 15 auuuli could be counted, an extra ring apparently having been split 

 off from the anterior sucker. 



The clitellum comprises 8 rings and is followed by 39 anuuli, each of which is 

 distinctly divided into two by a shallow groove, representing an intermediate stage in 

 ring multiplication not infrequently seen in Hirudinea. 



Thus there are, in all, 22 single auuuli followed by 39 double ones, behind the. 

 anterior sucker. 



The anterior half of the 39th (double) ring is the last completely to encircle the 

 body. 



In the absence of external metameric features, the ventral ganglia were exposed 

 and the somites plotted out, as seen in Fig. 1, according to the now generally adopted 

 iieuromeric standard. 



The typical or '' complete " somite is composed of three " primary " rings which, 

 as already stated, are sub-divided in the posterior region so that six " secondary " 

 rings can there he counted, a condition similar to that seen in the abdominal 

 ''complete" somites of Calliobdella. The clitellar somites, as is usual in the 

 Ichthyobdellidae, are modified in response, it would seem, to the comparatively bulky 

 reproductive organs crowded within them, which tend to displace the ventral ganglia 

 involved. 



Somite XI contains but two anuuli, and the anterior third of Somite XIII (con- 

 tained within the clitellum) appears to show the final stage in the history of a double 

 ring, the dividing groove, originally shallow, having deepened sufficiently to produce 

 two definitely single rings. 



The alimentary tract is shown in Fig. 2. The proboscis is relatively short ; the 

 intestine leaves the crop (stomach, thin-walled middle gut) in Somite XIX, 

 tapering gradually to the anus, which opens in the middle of the antepenultimate 

 double ring ; and special mention must be made of the last pair of crop diverticula or 

 caeca, which extend posteriorly beneath the intestine throughout nearly the whole of 

 its length. 



The extent of fusion, if any, which may exist between these caeca has been 

 regarded by Johansson (1898), in a valuable paper, as of considerable diagnostic 

 importance in the Ichthyobdellidae, and he cites a series of stages ranging from 



