472 PROCEEDINGS OF SECTION D. 
which is closest to the body, and has very few, or none. The 
posterior arthrobranch of segment 7 is reduced to a papilla, 
bearing some 9 or 10 gill filaments. The pleurobranchs have 
no filaments on the inner side, which is closely applied to the 
body. 
The gill filaments are, in some cases, provided with distinct 
hooks at their free extremities, the distribution of which appears 
constant. All the filaments of the posterior arthrobranchie, 
and the pleurobranchie, are of this description. The anterior 
arthrobranchie bear hooked filaments only on the inner surface, 
while in the podobranchie they are confined to the projecting 
lamina. 
Sense organs.—As in Astacus, the exopodites of the antennules 
bear setze, supposed to be olfactory in function. There is one set 
on each joint, not two as in Astacus, and it consists of a trans- 
verse row of three or four hairs, each of two joints, the distal one 
very much flattened. These are supported by a pair of sete, 
which are larger, and taper gradually towards the free extremity. 
Of the internal organs the circulatory and reproductive systems 
are most modified. 
Circulatory system.—The heart, lying in the same position as 
in Astacus, has three pairs of valves leading from the pericardium. 
Two pair are placed towards the dorsal side at the anterior and 
posterior ends. The third is at about the middle of the ventral 
surface. The arteries arise as in Astacus, and, except the sternal 
artery, follow the same course. The sternal artery divides into 
twoa short distance below the heart ; the branches encircle the 
intestine, and again unite just above the endophragmal skeletal 
plates, after which they follow the same course as the single 
sternal artery of Astacus. 
Reproductive system.—The ovary lies principally just in front of 
and below the pericardium. There is a median part between the 
pericardium and the stomach, from which project backwards two 
small lobes lying close together beneath the heart. In front it 
rises towards the dorsal surface, and there divides into two 
divergent halves. A long, narrow, tube-like prolongation of each 
half runs forward for some distance. At its extremity, between 
the large muscle of the mandible and the eye, it dilates into a 
small sack containing ova. The oviducts arise from the posterior 
end of the main undivided region of the ovary. 
The testes lie in the same position as the ovary, in front of and 
below the heart. They are formed of two lateral halves united 
together, only in the region of the vasa deferentia, about one- 
third from the anterior end. The posterior end is not formed of 
a median lobe as in Astacus, but of two distinct from each other, 
and often unequal in size. 
