208 Part I11.—Eleventh Annual Report 
Posterior antenne four-jointed, joints subequal in length, but the last 
two are more slender than the first and second; the apex is furnished ~ 
with three claw-like and hooked spines, and also a seta of about the same 
length as the spines. Mandibles nearly as in Lachomolgus forficula, 
Thorell. Basal part of the anterior foot-jaws stout, terminal part ex- 
tremely long and slender, gradually tapering to a setiform extremity, and 
bent at nearly right angles to the basal part; the upper edge of the ter- 
minal part is furnished with a fringe of cilia which extend from the geni- 
culation, where they are stout and setiform, but rapidly decrease in size 
towards the extremity. Posterior footjaws in the female rudimentary, 
three-jointed, the last very small, and without any spines or sets; the 
posterior foot-jaws in the male form powerful grasping organs, composed 
of two stout joints, and a very long falciform terminal claw, which carries 
a small seta at its base; the second joint has the distal half of the inner 
edge fringed with cilia (fig. 19). Both branches of the first four pairs of — 
swimming feet three-jointed, and of nearly equal length; the first basal 
joint of the first pair is furnished interiorly with an elongate and stout 
plumose seta, the inner margins of both branches are also provided with 
long plumose sete ; the armature of the inner branch comprises one seta 
on the distal end of the inner margin of the first joint, two on the second, 
and two on the third joint ; there are also four short dagger-shaped spines 
round the end of the third joint, and the outer distal angles of the first 
and second joints form tooth-like processes ; there is no seta on the inner 
edge of the first joint of the outer branch, one on the second and five on 
the third joint ; the third joint is also armed on the cuter margin and 
end with four dagger-shaped spines—the terminal one being the largest ; 
the second also bears one dagger-shaped spine on the outer distal angle, 
and the first jomt one. The armature of the fourth pair differs from that 
of the first in the following manner—there is no seta on the interior edge 
of the second basal joint, the first and second joints of the inner branch 
bear each a long plumose seta on the distal end of the inner margin, there 
is no seta on the third joint ; this joint is truncate at the end, and armed 
with two stout dagger-shaped terminal spines ; the outer distal angle of 
the second joint forms a bifid toothed process, and there is a small tooth 
on distal end of the outer margin of the first joint; the second joint of 
the outer branch bears one long seta on the inner margin, and the third 
joint five seta; the third joint is also armed at the extremity with a long 
sabre-like spine, ciliate along the inner edge, and with two short dagger- 
like spines on the outer margin ; and the first and second joints are each 
provided with a similar spine on the outer distal angle (fig. 21). The 
fifth pair are small and one-jointed, the apex truncate, and furnished 
with one long, stout, and slightly curved spine, and a small spiniform 
seta, both of which are plain (fig. 22), fifth pair alike in both sexes. 
First abdominal segment in both sexes considerably dilated ; that of the 
female has the sides rounded, is widest across the middle, and furnished 
on each side, on the ventral aspect, with a small setiferous appendage ; 
that of the male is widest across the distal end, the distal angles are 
somewhat produced, and provided with three small setz. The second, 
third, and fourth abdominal segments have the posterior margins in both 
sexes strongly setose. Caudal stylets about equal to twice the length of 
the last abdominal segment, and furnished with a small seta on the 
proximal half of the outer margin, and with four apical sete; the male 
abdomen is rather smaller than that of the female. Ovisacs two, large. 
Colour of the animal, including ovisacs, usually brilliant red. 
Habitat.—Living as a messmate within the shell of the ‘ horse mussel,’ 
Mytilus modiolus. Frequent in the Firth of Forth. I have also obtained 
Modiolicola in Mytilus modiolus, both in the Moray Firth, on the East 
