GIGLIOLI, ON THE GENUS CALLIDINA. 239 
the digestive and perivisceral cavities of Gammarus Pulex, in 
search of Gregarine, I first came across this species. At first 
I thought that I had got hold of a second entozoic Rotifer, 
and some time elapsed before I discovered my error, and 
that, instead of infesting the interior, it occurs as an epizoic 
parasite on the thoracic and abdominal appendages of Gam- 
marus Pulex and Asellus vulgaris, inhabiting chiefly the 
branchial plates. It fastens itself on the bodies of these fresh- 
water Crustaceans by means of the two suckers placed at the 
extremity of its last caudal segment; it often changes place, 
crawling over the body of its victim in a leech-like manner. 
I have found this species in no other situation, not even on 
other freshwater animals or on aquatic plants; and though I 
have examined about seven or eight hundred Gammari from 
different localities, I have not found one free from these Cal- 
lidinee. On Asellus they are not so constant. 
The body of C. parasitica is fusiform, and may be divided 
into a head, a neck composed of two false segments, a body 
or trunk consisting of one segment, and a caudal termination 
composed of six false joints (Pl. XI, figs. 1, 2). The penul- 
timate caudal segment terminates in a pair of moderately 
large claspers ; the last one terminates in a point, and sup- 
ports two soft, cylindrical, protrusible appendages, which 
terminate distally in a sucker (fig. 8). In a specimen I 
measured they were =!,,th of an inch in length; that of the 
claspers being =,),,th of an inch. 
The body is very transparent and colourless; no angular 
prominence exists on its central part, as in C. bidens ;* the 
caudal extremity is comparatively long. C. parasitica does 
not swim much, as the preceding species, but it creeps a good 
deal, the proboscis and calear being extended while the 
trochal disc is retracted. Its movements are precisely like 
those of a leech. It creeps in the following manner :—the 
suckers at the extremity of the tail fix themselves, the body 
is then stretched to the utmost, and its anterior portion is 
fixed (how, I was unable to make out) ; the tail is now drawn 
up, and the body contracted. All this goes on with great 
rapidity. The outer chitinous integument of the body or 
trunk is often thrown into strongly marked longitudinal 
rugze ; they generally appear when the creature diminishes 
the width of its body. 
The C. parasitica differ much in size; the largest adult 
one I measured, when fully extended, was —1,th of an inch in 
length and =4,th of an inch in breadth; the smallest was 
* P. H. Gosse, MSS., vol. iii, 1849, p. 9. 
