620 THE ANIMAL KINGDOM. 



branched stems are replaced by simple stems in B. minor. Scattered 

 ciliated cells occur in the epithelium of the caeca in Echiurus Pallasii and 

 Thallasema erythrogrammon. It is possible that these structures serve 

 two functions, (i) the removal of an excess of water from the coelome, 

 (2) the formation of excretory products by the epithelium of the caeca 

 themselves 1 . Large granules are found in their lining cells in B. minor, 

 and coloured drops or granules are massed in the groups of larger cells 

 observable in Echiurus Pallasii. 



A vascular system is absent in Priapulidae and a few genera of 

 Sipunculidae and Echiuridae. When present it is a closed system. In the 

 Sipunculidae it usually consists of a dorsal contractile vessel closely ap- 

 plied to the fore-part of the digestive tract ; but in some species of 

 SipunculuSy e. g. S. nudus, a second similar vessel is applied to the ventral 

 aspect of the same region. There is in many Sipunculids a peripharyngeal 

 vascular ring or plexus. Vessels from the ring or plexus enter the ten- 

 tacles, but in the lobed tentacles of Sipunculus they are replaced by a 

 plexus. Branches have also been observed passing from the dorsal vessel 

 to the rectum in some cases. Caeca, simple or slightly branched, may be 

 appended to the dorsal vessel in a single or double row. Ciliated cells are 

 found here and there in the epithelial lining of the vessels, or a band of 

 ciliated cells in the dorsal vessel. In the Echiuridae, starting from a 

 peri-intestinal ring or sinus (supra, p. 619), a vessel courses along the dorsal 

 surface of the oesophagus and pharynx to the apex of the prostomial lobe. 

 Here it bifurcates ; a branch lies on either side of the prostomium, surrounds 

 the pharynx, and then the two fuse and form a ventral supra-neural vessel 

 which ends blindly at the posterior end of the nerve-cord. A neuro- 

 intestinal vessel connects the ventral vessel to the peri-intestinal ring. The 

 origin of the dorsal vessel is sometimes dilated, and its walls muscular. 

 Contractions were observed in it by Greeff 2 . 



The coelomic fluid is sometimes colourless, but milky, as in Priapulus 

 and HalicryptuS) sometimes coloured pink or red. Amoeboid corpuscles 

 are always to be found in it, and appear to be the sole kind possessed by 

 Echiiirus. Spherical or biconcave cells coloured, or, as in Priapulus and 

 Halicryptus, colourless, may be present as well. There appear to be no 

 less than three blood pigments occurring in Gephyrea ; a brownish pig- 



1 The cilia of the funnels are said to cause a current solely outwards from the coelome in 

 Bondlia viridis (de Lacaze Duthiers). The asserted absence of funnels in some instances is probably 

 due to the examination of specimens preserved in spirit without previous preparation. The anal caeca 

 in question probably acquire their connection with the rectum. See account of male Bonellia and 

 the development of Echiurus, p. 623, and p. 624. In this case they are to be regarded as homologous 

 serially with the anterior nephridia described further on. 



8 Danielssen and Koren ascribe to Hamingia arctica a vascular system differing in some respects 

 from the account given in the text ; but Horst finds in H. glacialis (probably the same species) a 

 system wholly conformable to it. 



