51 6 PYCNOGONIDA ' CHAP. 
Circulatory System.—The heart has been especially studied 
by Dohrn in Phowichilus. It consists of a median vessel running 
from the level of the eyes to the abdomen, furnished with two 
pairs of lateral valvular openings, and sometimes, though not 
always, with an unpaired one at the posterior end. The walls 
are muscular, but with this peculiarity that the muscular walls 
do not extend around the heart dorsally, in which region its 
lumen is only covered by the hypodermis and cuticle of the back. 
The blood-spaces of the body are separated into dorsal and ventral 
halves by the septal membrane already referred to, which is per- 
forated in the region of the lateral processes by slits placing the 
two cavities in communication; this septal membrane runs through 
the limbs to their tips, and far into the proboscis, where it is 
attached to the edge of the superior antimere. The blood is a 
colourless plasma with several kinds of corpuscles, of which the 
most remarkable are amoeboid, actively mobile, often coalescing 
into plasmodia. The course of the circulation is on the whole 
outwards in the inferior or ventral sinus, inwards towards the 
heart in the superior, save in the proboscis, where the systole of 
the heart drives the blood forwards in the dorsal channel. The 
beat is rapid, two or three times in a second, according to Loman, 
in Phoxichilidium. Especially in the species with small body 
and exaggerated legs, the movement of the circulatory fluid is 
actuated more by the movements of the limbs and the contrac- 
tions of the intestinal caeca than by the direct impulse of the 
heart. 
Nervous System.—The nerve-chain consists of a fused pair 
of supra-oesophageal ganglia, which innervate (at least in the 
adult) the chelophores, and of ventral ganglia, whence proceed 
the nerves to the other limbs. The ganglia of the second and 
third appendages are fused with one another, sometimes also 
with the ganglia of the first ambulatory legs; the ganglia of the 
three posterior pairs of legs are always independent (though the 
development of their longitudinal commissures varies with the 
body-form), and they are succeeded by one or two pairs of 
ganglia, much reduced in size, situated in the abdomen, of which 
the posterior one innervates the muscles of the abdomen and of 
the anal orifice. Each lateral nerve divides into two main 
branches, which supply the parts above and below the septal 
membrane. The nerve-supply of the proboscis is very com- 
