56 THE VOYAGE OF H.M.S. CHALLENGER. 
circular in form, and about 2 mm. in diameter ; the form of the cirri, which are, however, 
both relatively and absolutely larger, is like that of Myzostoma quadrifilum. 
In the possession also of six caudal appendages it is intermediate between that form 
and Myzostoma. \ Jissum. The form of these appendages is like those of Myzostoma 
quadrifilum; they are divided into a larger basal part and a finer terminal thread 
( CAx-CAg ). The last appendage ( CA 3 ) that was not damaged measures from the margin 
of the body to the extremity 1‘3 mm., the basal and terminal parts being of equal length. 
The second pair (OA 2 ) is somewhat shorter, and the exterior pair ( GA X ) shorter still. 
The base of all the six appendages is continued for some way into the ventral surface 
of the disk. The slight development of the last pair of cirri (c.) is remarkable, compared 
to the rest, — they measure only '45 mm. 
Host. — Antedon midtiradiata , P. H. C., from Station 187 (Torres Strait) of the 
Challenger Expedition. 
51. Myzostoma quadrifilum, n. sp. (PI. IV. figs. 3-6). 
I had thirteen specimens, which I have united together under this specific name, and 
which came from the same host. The body is roundish, and terminates in four filiform 
caudal appendages, which differ from those of Myzostoma quadricaudatum in having a 
terminal thread. The light yellow marginal zone is distinctly marked off from the brown 
opaque middle portion. All the specimens, however, are not absolutely the same in 
regard to the length of the caudal threads and the form and length of the cirri ; in some 
individuals the caudal appendages are entirely separated from their origin, in others they 
are united for a certain distance. There are, for example, two specimens measuring 
‘9 mm., in one of which the cirri are long, narrow, and slender, *27 mm. long, and 
appear to be direct continuations of the margin, while in the other (figs. 5, 6) they are 
short, stout, and much wrinkled, measuring only T mm. in length, and arise from the 
under side of the disk by a thick basal portion. I was prevented from making a strict 
distinction between the different varieties, from the fact that all the specimens, except 
those shown in figs. 3 and 4, were more or less injured or rolled up, and therefore 
unsuitable for an exact examination. 
The smaller of the figured specimens (fig. 4) had a circular form, and measured ‘7 nun., 
considering, as in Myzostoma quadricaudatum, the cloaca to be the termination of the body. 
The distinction between the disk and the caudal appendages is in this specimen very slightly 
marked, inasmuch as the basal parts of all the cirri are grown together, and the summits 
only are distinct. The intestinal caeca (i.) penetrate into the base of the caudal appen- 
dages — one into each. The mouth is subterminal; the cloacal opening (cl.) is placed on 
a slender papilla lying between the bases of the median caudal appendages. The pharynx 
(ph.) is moderately large, but I was unable to examine it in detail. The oval suckers ( s .) 
are similar to those of Myzostoma quadricaudatum, and distant from the margin about 
