CIRCULATION. 



There are as yet many contradictory views regarding the circulatory 

 system. Delle-Chiaje, who was the first to describe the circulation of the 

 Echini, entertained somewhat erroneous views ; the subsequent observations 

 of Tiedemann have in the main been confirmed by Agassiz, Valentin, and 

 Miiller. In the Desmosticba we have a genital ring surrounding the anal ex- 

 tremity of the alimentary canal, which sends out, near the madreporic body, a 

 branch leading towards the jaws, following the course of the stone canal and 

 opening into the heart (an ovoid canal), from the base of which it emerges 

 again to form a circular ring round the oesophagus, and then follows the 

 inner edge of the alimentary canal as the alimentary artery, while on the 

 outer edge of the alimentary canal runs what we might call the alimentary 

 vein, connected with the heart, sending out branches to the test and to 

 the walls of the alimentary canal. No vibratile cilia have been detected in 

 the circulatory system. In addition there is, according to Valentin, a branch 

 of both these systems extending along the ambulacra] tubes, but I have been 

 unable to trace it. The heart is an ovoid canal surrounded by a vibrating 

 membrane, a sort of pericardium. 



The same general course of the circulatory system is traced in the Clype- 

 astroids ; we find the inner and outer vessel following the course of the ali- 

 mentary canal (PL XXXVII. f. 1 ; PL XXIX. lower fig.), with a well-devel- 

 oped oesophagal and genital circular ring (PL XXIX.). 



In the Petalosticha, the circulatory system does not follow as intimately 

 the course of the alimentary canal, but is carried partly upon the mesenteric 

 plates connecting the windings of the alimentary canal (PL XXXII f. 17,18). 

 Hoffmann has succeeded in proving for Spatangoids the existence of a con- 

 nection between the water system and the circulatory system (PL XXXII 

 f. it), which had formerly been shown, by Agassiz, to exist in Echinarach- 

 nius. This connection has not as yet been traced in Desmosticha. though it 

 undoubtedly does exist also, as blood cells have been found in the water 

 system. The course of the actinal vessel on the actinal floor, with a branch 

 leading along the anal extremity of the alimentary canal, as well as its 



