PYCNOGONIDA 783 



which is often the case in deep-sea species, the tubercle persists. 



The chitinous cuticle which covers the body is provided with 

 numerous cavities which open to the exterior by narrow canals. 

 There are no specialized respiratory organs and Hoek holds 

 the view- that these cavities assist in the necessary exchange 

 of respiratory gases. The cuticle also bears bunches of bristles, 

 probably sensory. It is lined by a nucleated, protoplasmic 

 layer which represents the ectoderm. Numerous glands open 

 to the surface, especially on the palps and on the ovigerous legs 

 where their secretion serves to attach the eggs in the male 

 during the breeding season. 



The nervous system consists of a chain of four or five ganglia. 

 The first or supra-oesophageal gives rise to the nerves which 

 supply the mandibles or first pair of appendages and the eyes, and 

 to an azygos dorsal nerve which runs to the proboscis and there 

 forms a ganglion. The first sub-oesophageal ganglion sends off 

 a pair of nerves to the proboscis, each of which also forms a 

 ganglion, and the three ganglia in the proboscis are connected 

 by a periproboscidial nerve ring. From the sub-oesophageal 

 ganglion two more pairs of nerves arise, the anterior of the two 

 run to the palps or second appendages and the other to the 

 ovigerous legs. This ganglion consists in the young forms of two 

 separate ganglia which subsequently fuse. The sub-oesophageal 

 ganglion is followed by a chain of four or more (rarely of three) 

 thoracic ganglia, which supply the four pair of legs and give 

 off nerves posteriorly to the rudimentary abdomen. 



The proboscis is traversed by an oesophagus lined with a 

 chitinous lining which is produced into spines and teeth and 

 plates. The latter arc conspicuous in the enlargement which 

 occurs in the oesophagus before it leaves the proboscis and, as 

 numerous muscles run from them to the wall of the proboscis, 

 it is not unlikely that they act as a suctorial apparatus. Behind 

 the nerve-ring the oesophagus projects into the stomach so as 

 to form a valve and in this region certain glands are described. 

 The stomach is beset with glandular villi and it gives off a certain 

 number of tubular digestive caeca which may extend into the 

 proboscis and which do extend into the legs and mandibles even 

 as far as the sixth joint. A short rectum leads to the anus which 

 in Colossendeis is laterally placed. 



There is a dorsal heart with usually three pair of ostia and 



