EEPOET ON THE MYZOSTOMIDA. 
63 
Sir Wyville Thomson, and a number of portions of arms of Pentacrinus. This species does 
not produce real cysts upon the arms of its host, but only swellings of several (3-6) joints, 
which gradually disappear. In fig. 9 there is displayed a swollen arm viewed from the 
dorsal surface and from the riglit and left sides. The swollen segments of the arms are not 
placed so close together as in the normal healthy arm, and are irregular in shape, being 
sometimes partially replaced by intercalated plates ; the basal pieces of the pinnules that 
arise from these joints are considerably enlarged. The cavity where the Myzostoma lives 
lies between two neighbouring joints ; it opens on to the exterior by a slit which is bordered 
in an irregular fashion. If more than one Myzostoma be associated together in the same 
malformation, then the cavity of this is divided up into secondary cavities, one for each 
parasite. Thus in fig. 9 the swollen portion of the arm shows two cavities, separated from 
each other by a thick calcareous wall, and each opening by a separate aperture and ending 
in a tube-like prolongation on the ambulacral furrow. Fig. 10 represents the arm (fig. 
9(7) cut in the direction of the arrow and viewed from the distal end. In each cavity is a 
Myzostoma, with the anterior end turned towards the tube-like prolongation on the am- 
bulacral furrow, and the pharynx extends out of the aperture. The swelling in one of 
the specimens in my possession occupied only two joints, a third joint being only slightly 
malformed; there was but one aperture on the dorsal side (fig. 15), and the cavity con- 
tained but a single specimen. The expression “ absorption of the calcareous matter ” 
used by v. Willemoes Suhm is not, in my opinion, happily chosen. There is in fact no 
absorption, but only a transference of the calcareous matter ; it disappears inside in con- 
sequence of the growing Myzostoma, but is deposited again peripherally, and increases 
the thickness of the arm. Nor are the arms so brittle at the place occupied by the 
parasite as his description would seem to indicate ; they appeared to me to possess as 
great a resistance as any of the other parts ; the malformations, however, must interfere 
seriously with the mobility of the arm. 
This Myzostoma (figs. 12, 13) is of a uniform brown colour, and like all the encysted 
forms has its lateral portions turned upwards, so that sometimes the marginal borders lie 
one upon the other (fig. 12) ; sometimes the dorsal surface appears to be reduced to 
a longitudinal furrow, so great is the folding of the sides of the body (cross section, fig. 
14). Inside the cavity, the animal is placed with its ventral side turned towards the 
axis of the arm of the Pentacrinus, the dorsal fissure towards the periphery of the 
cavity, occasionally directly (fig. 9 B) towards its opening. 
The animal is rolled up in such a regular fashion that it becomes tube-like ; from one 
end (fig. 13) projects the pharynx ( yjh .), and from the other the cloacal papilla {cl.), both 
being terminal in position. The parapodia (p.) are placed on each side in a semicircle, 
between the centre and the margin of the body ; they are small, conical warts of '2 mm. 
in length, and lie in shallow cavities. 
Suckers are entirely absent. The form of the body when unrolled is circular, and 
