200 
W. A. HAS\YELL. 
claspers, which are of essentially the same character in both. 
The posterior limbs do not differ in any essential feature in 
the two species, except that the cirrus (figs. 18 and 19) with 
which it is provided is divided into two segments by means 
of a joint like the other cirri, the corresponding appendage 
in S. tasmanicus being undivided. 
AYith a close agreement in structure between the Australian 
(continental) and Tasmanian forms there is associated a 
correspondingly close resemblance in movements and general 
mode of life (see 6, p. 303). In ordinary locomotion the 
anterior limbs are usually completely retracted, progression 
being effected exclusively though the peculiar ‘‘walking 
movements of the hind limbs. Frequently the animal remains 
for a long time fixed by means of the hind limbs, the body 
being either extended fiat on the surface, the head groping 
about with the tentacles and occasionally “ grazing on the 
substratum by series of rapid movements of the jaws, or 
assuming a half-erect position with the tentacles fiexed 
towards the dorsal side. 
Stratiodrilus is able to walk in a reversed position by 
means of the surface film of the water. But much more 
remarkable is its ability to progress in mid water. The 
movement can hardly be described as swimming : slow 
locomotion through the Vvater is effected by deliberate 
alternate movements of the hind limbs, as when the animal 
is progressing on the surface of a solid object. This was 
observed only in water clouded with suspended fiocculent 
particles. 
Though the usual habitat of Stratiodrilus is the gill- 
cavity of its host, specimens are sometimes found on the 
outer surface. Probably this only occurs under abnormal 
conditions. 
Like Histriobdella, as stated by Shearer ( 9 , p. 297), 
Stratiodrilus readily deserts its host when conditions are 
in the least abnormal. When the crayfish is kept for a little 
time in a vessel with a limited amount of water, Stratio- 
drili will frequently be found crawling on the bottom of the 
