116 LECTURE X. 



Caecal appendages are not continued from the central stomach 

 into the rays of the Ophiura or Comatula. In the latter genus the 

 alimentary canal presents a higher type of structure : there is a 

 slightly convoluted intestinal canal which terminates by a distinct 

 tubular anus. 



Professor Tiedemann, in his celebrated monograph on the Echino- 

 derma, has successfully demonstrated the vascular system in all the 

 leading forms of that class. In the Asterias ruhens the vessels which 

 absorb the chyle from the digestive sac terminate, after a series of 

 reticulate anastomoses, in a circular trunk, which likewise receives 

 branches from the radiated caeca. The venous circle communicates 

 by means of a dilated tube, regarded as a rudimental form of 

 heart, with an arterial circle surrounding the mouth, from which 

 branches diverge to the rays and other parts of the body. I have 

 not been able to trace any direct communication between the true 

 vascular system of the Asterias and the system of canals, which, by 

 their connection with contractile divreticula, govern the supply of 

 fluid to the vesicles at the base of the hollow tentacles protruded 

 through the ambulacral pores. Tiedemann and Dr. Sharpey also 

 agree in rejecting the continuation of the erectile system of the feet 

 from the intestinal vascular system. 



There is a small tube, called by Tiedemann the sand-canal ; its 

 position is indicated by the circular prominence or nucleus on the 

 dorsal aspect of the disc of the Asterias, near the angle between two 

 of the rays, which prominence resembles a miniature brain-stone 

 madrepore. The problematical calcareous column in question is con- 

 tinued from the nucleus into the interior of the body, and consists of 

 minute hexagonal plates, which are united into larger joints. From its 

 analogy with the jointed column of the crinoid star-fishes, it has been 

 suggested by Dr. Coldstream that it may be the analogue or rem- 

 nant of that column ; but, according to the observations of M. Sars, 

 the Asterias are not fixed animals in the young state. Dr. Sharpey has 

 conjectured that it may serve as a filter in the admission of sea water 

 to the tubular system of the ambulacral feet. Such a mechanism is 

 not, however, present in the Echini or Holothurice, which equallj'- 

 possess the systems of tubular feet. 



As the sea water is freely admitted into the general cavity of the 

 body, and bathes all the viscera, their vascular surfaces thus stand 

 in the relation of a respiratory organ to the aerated medium, and 

 they are every where provided with vibratile cilia, which maintain the 

 currents of oxygenated fluid.* 



The nervous system of the Asterias (p. \^, Jig. 4.) consists of a 



* Sharpey, in Cyclopeedia of Anatomy and Physiology, art. Cilia. 



