41 6 GEPHYREA CHAP. 



A similar but shorter tube is found on the ventral surface of the 

 anterior end of the alimentary canal in the species in question ; 

 it also opens into the ring which surrounds the mouth. 



Respiratory System. There are no special respiratory 

 organs, and it has long been a matter of dispute where the 

 respiration of Gephyrea is carried on. The oxygenation of the 

 blood probably takes place to some extent through the walls of 

 the oral fringe, but the blood which receives its oxygen at this 

 spot is limited in its distribution, and could only supply the 

 brain and head. It seems probable that the remaining organs 

 are supplied with oxygen by the fluid of the body-cavity, which 

 bathes them on all sides. This might obtain its oxygen from 

 the blood in the heart, or more probably, through the thin walls 

 of the intestine, from the stream of water which is maintained by 

 the ciliated groove described above. Quite recently a form S. 

 mundanus, var. branchiata has been described l with thin-walled 

 papillae covering parts of the skin. These papillae are full of 

 corpuscles, and are regarded by their discoverer as branchiae. 



Body-Cavity. The pinkish fluid of the body-cavity contains 

 numerous corpuscles, the products of the reproductive organs 

 (either ova or spermatozoa), and some curious unicellular bodies 

 known as " urns." The latter are shaped like a bowl with a 

 ciliated rim, and are formed from the budding of certain cells on 

 the walls of the dorsal blood-vessel. 2 Their function is unknown, 

 but they resemble certain multicellular bodies found in the body- 

 cavity of Phascolosoma. The generative cells found in the body- 

 cavity are further considered below. The true corpuscles are 

 either biconcave round corpuscles coloured with a chemical sub- 

 stance, the haemerythrin of Krukenberg, which apparently plays 

 the same role as haemoglobin in other animals ; or amoeboid 

 corpuscles, which, though rare in Sipunculus, are very numerous 

 in Phascolosoma, 



Nervous System. The nervous system of Sipunculus con- 

 sists of a brain or cerebral ganglion, a circumoesophageal ring 

 surrounding the gullet, and a ventral nerve-cord. The brain is 

 a small bi-lobed nervous mass situated on the dorsal surface of 

 the oesophagus, in the angle between the right and left dorsal 

 retractor muscles close to their point of insertion. Numerous 



1 Fischer, Abh. Ver. Hamburg, Bd. xiii 1 . 1895, p. 1. 

 2 Cuenot, Arch. Zool. exp. (2) ix. 1891, p. 693. 



