216 THE VITAL FUNCTIONS. 



stem without any visible cause determining these 

 movements. Simihir phenomena have been ob- 

 served in CampanidaricE and PlumularifE* 



The movements of fluid now described are partial 

 and irregular, and occur but to a limited extent. 

 The first rudiments of a vascular apparatus among 

 Zoophytes present themselves in the Echinoder- 

 m(tla.'\ Asterias, Echini, and Holothurise, are fur- 

 nished with a delicate network of veins, spread 

 over the membrane of the intestine, receiving 

 through it the nutrient juices, and collecting them 

 into a large reservoir, or vessel, which passes, in 

 the form of a ring, round the upper part of the 

 alimentary canal, and is connected by a vertical 

 tube with a similar vascular ring, surrounding the 

 mouth, which is below. From this lower ring there 

 are sent off, in radiating lines, arterial trunks, 

 generally five in number, corresponding to the 

 compartments into which the framework of the 

 body is divided. J The oxygenation of the blood 

 is effected, in the course of its circuit, by means of 

 water admitted into the internal cavities, over the 



* Lister; Philosophical Transactions, 1834, p. 365. These cur- 

 rents of fluid were first noticed by Cavolini. 



t Ehrenberg, indeed, supposed that he had found traces of blood- 

 vessels in some of the Rotifera, and more particularly in the Hyda- 

 tina: but these appearances are now generally believed to arise 

 from the presence of muscular bands only. 



X It has not vet been ascertained whether the vesicular pedicles 

 of the Echinodermata are injected with the same fluid which circu- 

 lates in the blood-vessels, or with some other fluid contained in 

 another set of vessels having no communication with thei former. 

 Delle Chiaje regards both kinds of vessels as forming one continuous 

 system : while Tiedemann considers them as being totally uncon- 

 nected with one another, and as containing fluids of very different 

 kinds. 



