HARDING—ON A NEW LAND-LEECH FROM THE SEYCHELLES 43 
blood-sucking species with fully armed jaws to those of carnivorous habit in which these 
features are more or less vestigial or even absent. 
Whatever interpretation be placed upon this scale of facts, whether it be read 
upwards or downwards, one conclusion is the same: the change to which it bears witness 
has been a gradual one. 
But no intermediate stages are known to exist between the Trignathoferze and the 
Duognathoferee, and it seems, therefore, not unreasonable to suppose that the dis- 
appearance of the third jaw occurred suddenly; in short, that we have here an instance 
of mutation. 
Again it is difficult to understand how the presence of only two jaws, capable of 
making a wound one-third less in size than that inflicted by three jaws, can be of any 
benefit to a blood-sucking parasite and the doubt is increased by a consideration of the 
comparative scarcity of the species provided with the lesser armament and the enormous 
multiplication of the three-jawed forms. 
The mutation, then, has not been to the advantage of the race and the Duognathoferze 
are to be regarded as one of Nature’s unsuccessful experiments. 
We know little, however, of the anatomy, still less of the distribution, and nothing 
at all of the embryology of many of the known species of Haemadipsine ; other forms 
probably await discovery, and further speculation seems idle in the present state of 
our knowledge. 
In conclusion, Idiobdella seychellensis is not merely at a disadvantage in respect of 
its armament. In the only island in which it has so far been found there are no terrestrial 
warm-blooded animals upon which it can prey and it probably sucks the blood and juices 
of earthworms and other soft-bodied invertebrates. 
It is the smallest of the Hemadipsine and its diminutive size, its reduced number of 
testes, its lack of auricule, and its extreme scarcity substantiate the view that it is 
a degenerating species in process of extermination. 
EXPLANATION OF PLATE 6. 
Fig. 1. Idiobdella seychellensis, from a specimen preserved in alcohol, dorsal view. x 4. 
Fig. 2. The same, ventral view. x 4. 
Fig. 3. The same, dorsal view of head region, showing the eyes. x 16. 
Fig. 4. The same, dorsal view of posterior extremity. x 16. 
Fig. 5. Ventral view of anterior region. The anterior sucker and part of the body posterior to it have 
been slit up by a median incision exposing the two jaws. Greatly enlarged. ce. cut edge of body 
wall; pe. entrance to pharynx; 7. jaw; s. interior of sucker. 
Fig. 6. Reproductive organs. Greatly enlarged. The parts, which originally were tightly packed 
together, have been opened out for the sake of clearness and the posterior pairs of testes are 
omitted. ps. penis sac; ep. epididymis; v.d. vas deferens; ¢1, ¢2, £3, first, second and third pairs 
of testes ; ov. ovary; p.ovd. and c.ovd. paired and common oviducts; v. vagina; vg.d. vaginal duct to 
exterior. 
