EEPOET OH THE MYZOSTOMIDA. 
65 
cyst was lined by a tough brownish membrane, which in the middle of the cyst (in the 
direction of the arrow, fig. 2) on the antambulacral side is raised into a fold half as high 
as the cavity. Two equal-sized cavities communicating by a fissure are thus formed, in 
each of which was a Myzostoma , which did not, however, occupy the whole of the cavity ; 
each lay on the ventral side of the cyst, at opposite extremities of the cyst, one near the 
point the other near the arm-joint; the proximal chamber (fig. 3) communicates with the 
exterior by a fine foramen. The second cyst had the same form and size, the third was 
similar but smaller by one-third, and "was formed of numerous polygonal calcareous 
plates, the ambulacral furrow being bordered by several rows of them. The two parasites 
occupying this smaller cyst were, although small, evidently adult, for they were filled 
with eggs. This small cyst also enabled me to come to some conclusion respecting the 
growth and formation of the cysts. In all probability its growth commences by an 
enlargement of the joints of the pinnule ; additional plates are subsequently intercalated, 
which, finally, when the parasite is fully developed, fuse together and form a solid wall, 
showing only traces here and there of its original composition (figs. 2, 3, 4, 6). The cyst 
displayed in figs. 4-6 (its point is broken off) differs from the others by its more 
rounded form, and by the two neighbouring arm -joints, as well as the one that bears the 
pinnule, becoming swollen. The hinclermost of these (c.) has an enlarged pinnule— here 
broken off — whereas the anterior has lost its pinnule. The septum dividing the cavity of 
the cyst is longitudinal, it is attached to the wall only near the external orifice, and there 
is a fissure therefore left putting the two cavities into communication, but too small to 
allow the parasite to change its place. As in fig. 3 so also here (see the arrow, fig. 5), 
the aperture communicating with the exterior belongs to one of the two cavities only. 
This species is found always in pairs in a single cyst, the individuals being of the 
same size and of a dark brownish colour and peculiar form (fig. 1). The lateral parts, 
as in Myzostoma pentacrini, are turned upwards at a sharp angle, so that in section 
the body is wedge-shaped (figs. 7, 8) ; the flat back of the wedge is formed by the 
ventral surface of the animal, and is bordered by the parapodia (fig. 1 B ) ; the parts lying 
outside the parapodia are bent towards each other and form the sides of the wedge. The 
marginal borders being unprovided with cirri are apt occasionally to come into such close 
contact above that only a very narrow fissure is left, through which the dorsal surface 
of the animal can be discerned ; sometimes the marginal borders are again bent outwards 
(fig. 1 A) in a lip-like fashion, so that a kind of tray is formed, the outer part of which 
corresponds to the ventral, the inner to the dorsal side of the animal. 
Suckers are entirely absent, and the five pairs of parapodia are but slightly promi- 
nent as flat circular disks. The hook-apparatus is very feeble in proportion to the size of 
the animal, and the hook is only about half as long as the thickness of the body (fig. 7, u). 
The length of the specimen shown in fig. 1 (taken from the cyst, fig. 2), is 3 mm., the 
breadth of the flat ventral side, 2 - 4 mm. Considering that the bent portions of the 
(ZOOL. CHALL. EXP. — PART XXVII. 1884.) D(1 9 
