34 
THE VOYAGE OF H.M.S. CHALLENGER. 
11. Myzostoma gigas, Liitken (PI. II. figs. 1-8). 
Myzostoma gigas, Liitken, MS. 
„ ,, Graff, Proc. Roy. Soc. Edin., vol. xii. p. 378, 1884. 
In the year 1877 I received from Dr. Liitken some Myzostomida from the Copen- 
hagen Museum, which on account of their large size had been named by him Myzostoma 
gigas. 1 
This species measures 7 mm. in diameter, the thickness of the body reaching 
1*5 mm. in the middle ; the back is convex, increasing gradually from the margin, and 
the animal appears therefore lenticular ; the ventral surface is flat. At the extreme edge 
the body is fransparent, and becomes more and more opaque towards the centre. The 
dorsal surface is covered with tubercles, distinguishable by the naked eye, and separated 
from each other by intervals (fig. 3), which become smaller and smaller, and finally 
disappear at the margin of the body. The latter is provided with twenty long, equal 
cirri, measuring T6-'2 mm., and arranged at equal distances from each other, except 
the first pair, which are rather further removed from each other than the succeeding 
cirri. Each cirrus arises in a small marginal notch. The parapodia (figs. 1, 2, p.) are 
stout, and arranged in a circle at equal distances from the centre and the periphery. 
Each parapodium has a number of annular furrows and a strong terminal hook-apparatus. 
The hooks, which are furnished with a fine somewhat bent tip (figs. 4, 5, 6, u.), are from 
•2-*26 mm. long, and ‘01— '02 mm. thick. The manubrium (ma.) is a convex plate, with five 
or six digitiform processes on its free margin. The inequality in length between the stalks 
of the manubria is very striking. Two of them taken from the same individual measure 
respectively ‘26 and T5 mm. in length. The suckers (s.) are hemispherical, and very 
large, and situated between the insertions of the parapodia and the margin of the body ■ 
they bear a number of incisions round the edge. The pharynx (ph.) is small in proportion 
to the body ; the mouth (m.) and cloacal papilla (cl.) lie at the same level as the suckers, 
at a considerable distance from the margin. 
The male genital papilla ( $ ) is also remarkably large. All the other specimens that I 
have are much smaller than those from the Copenhagen Museum, and differ also in their 
colour, which varies from light yellow to red-brown. The margin of some of the smaller 
specimens is also more transparent and more marked off from the rest of the disk, especially 
in a specimen from the Amsterdam Museum, collected during the Dutch Arctic Expedition, 
which comes nearest in size to the specimens in the Copenhagen Museum. The smallest 
specimens of all were gathered during the Challenger Expedition. One of these is 
displayed in figs. 7, 8 ; it was taken at Bahia, from Antedon carinata, and is of a 
1 Dr. Chr. Fr. Liitken, A revised Catalogue of the Annelida and other not Entozoic Worms of Greenland, 
p. 178, No. 120, Myzostoma gigas, Ltk. (MS.), in Manual of the Natural History, &c., of Greenland, edited by Prof. T. 
Rupert Jones, for the use of the Arctic Expedition, London, 1875. 
