116 AETHUR E. SHIPLEY. 



never seen it in this condition in those specimens of the un- 

 armed Gephyrea which I have been able to examine alive. 



The absence of hooks on the introvert is a marked feature of 

 this species of Phymosoma, Only five other species of the 

 genus are devoid of these characteristic chitinous structures. 



The Vascular System. 



The vascular system of the unarmed Gephyrea consists of a 

 closed space which has no capillaries in connection with it. 

 The system is distributed in the various parts of the head, and 

 its chief function would seem to be that of distending the ten- 

 tacles and lower lip. In the tentacular blood-vessels the blood 

 is separated from the surrounding water by a thin layer of 

 tissue, and it is very probable that it becomes aerated during 

 its passage through these organs. Its function as a carrier of 

 oxygen cannot be of very great importance, since the system 

 is entirely confined to one small part of the body. Probably 

 those organs outside the head are dependent for their aeration 

 on the corpuscles in the perivisceral fluid, though it is not easy 

 to see where these can get their supply of oxygen and elimi- 

 nate their waste matter. 



The large vessel which lies on the dorsal surface of the 

 oesophagus, and which is usually known as the heart, acts as a 

 reservoir into which the blood retires when the tentacles are 

 retracted (figs. 4 and 7). In Ph. varians, where there were 

 few tentacles, the heart was a straight blind sac about '5 cm. 

 long, extending along the dorsal side of the oesophagus ; but in 

 Ph. Weldonii the number of tentacles is much greater, and 

 the reservoir is correspondingly increased. The heart is much 

 longer, being continued along the dorsal side of the oesophagus 

 for a centimetre or two, and thus becoming involved in the 

 twisting of the alimentary canal (fig. 4). Its capacity is also 

 much increased by numerous small diverticula, which project 

 as finger-like processes, and give the heart a very characteristic 

 appearance. The walls of the heart and its diverticula are 

 thin, with few muscle-fibres in its substance. Similar diver- 

 ticula occur in the species Ph. antillarum, Ph. pelma, and 



