516 PYCNOGONIDA CHAP. 



Circulatory System. The heart has been especially studied 

 by Dohrn in PJioxichilus. 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- 



