6 
THE VOYAGE OF H.M.S. CHALLENGER. 
to be the ventral surface of the body, the presence of the central nervous system con- 
clusively proves it to be so, since this structure, as already stated; lies between the 
ventral muscular mass and the ventral wall of the body ; in this way the opposite side 
of the body bearing the mouth and cloacal aperture must be regarded as the dorsal 
surface, since the intestine never lies below but always above the central muscular mass. 
The two European species — Myzostoma glabrum and Myzostoma cirriferum — though 
they by no means represent the two extremes of the series, exemplify two groups which 
differ from each other in another respect. In the one the body is stout and massive and 
of a solid consistence ; the back is usually vaulted ; as a rule there are no cirri, or 
they are, if present, represented by short inconspicuous processes ; the ventral surface is 
bulged out by the strongly developed muscular mass, and from this central elevation a 
number of smaller ridges run to the bases of the parapodia, and between them still smaller 
ridges connect the central elevation with the suckers. These ridges appear to be pro- 
duced by the muscles of the parapodia — especially the musculus centralis — and the 
suckers ; two other ridges, one running forwards and the other backwards, correspond 
to the pharynx and cloaca. On the dorsal surface one sees very often five pairs of 
feeble elevations, which mark the position of the base of the hook-apparatus, and 
in Myzostoma testudo become very much enlarged. Sometimes [Myzostoma costatum) 
the whole intestine shows itself on the outside, in the form of a series of elevated ridges. 
In the other group the body is thin, flat, membranous, and somewhat transparent, 
with a more highly transparent marginal border, which is owing not merely to the slighter 
development of the muscles, but to the fact that the ramifications of the intestines and 
genital glands do not quite extend to the periphery of the animal (fig. 1). The greater 
delicacy of the body in this group is owing to the slighter development of the muscles of 
the body, especially of the ventral muscular mass ; but the presence of a more transparent 
marginal border is not confined to these forms. The second group is also characterised 
by the possession of long cirri or cirrus-like processes of the margin of the body. The 
number of these is not limited to twenty, but occasionally exceeds that number. 
There are, it is hardly necessary to say, numerous transitional forms, which unite 
the two groups, and can be with difficulty assigned to either ; among these are the 
very remarkable forms distinguished by the possession of two, four, or six finger-like 
caudal appendages such as Myzostoma lobatum and Myzostoma Jissum. 
Colour and Sculpturing of the Skin. 
As might be expected, the group containing the larger and stouter forms displays most 
variety in colour and sculpturing, though only on the dorsal surface — the ventral surface 
being always of a uniform dull yellow or brown. The second group are nearly all yellow 
or brown — the prevailing colour of the Myzostomida — with a somewhat lighter coloured 
