276 ECIIIXODERMATA. 



is often surrounded by projecting skeletal plates armed with 

 spicules. There may even be developed, as in the Cidaridea and 

 Clypeastridea, pointed teeth covered with enamel, constituting a 

 powerful masticatory apparatus, which again is supported around 

 the oesophagus by a system of plates and rods. This appagfrtus is 

 known as Aristotle's Lantern (fig. 214). In the Holothurians, on 

 the other hand, there is a calcareous ring round the oesophagus. It 

 is formed of ten pieces, and serves for the attachment of the longi- 

 tudinal bundles of the dermal muscles. 



In the Star-fishes the digestive canal is invariably short, sac-like, 

 and beset with branched blind appendages, some of which lie in the 

 disc, while some project a long way into the arms. In the Asteroidea 

 we find five pairs of strongly developed multilobed diverticula of 

 the middle division of the alimentary canal (fig. 218). The five 

 diverticula of the short rectum which fall in the interradii are 

 much shorter, and perhaps perform the function of kidneys, while 

 the longer diverticula increase the digesting surface. In the other 

 Echinoderms the narrow intei-tine is much increased in length, and 

 is either, as in Comatula, coiled round a spindle in the axis of the 

 disc, or, as in the Sea-urchins, describes some convolutions (fig. 217), 

 and is attached to the inner surface of the shell by fibres and 

 membranes. In the Holothurians also the intestine is, as a rule, 

 much longer than the body, and is usually folded upon itself three 

 times and attached by a sort of mesentery (fig. 219). 



The true vascular system is very difficult to trace. It consists in 

 most Echinoderms of a ring-like vascular plexus surrounding the 

 oesophagus. From this circular vessel radial vessels pass oft' one to 

 each ray, and these trunks again give oft' other branches. There is 

 in addition on the dorsal surface a second circular vessel, which sends 

 off branches to the stomach and generative organs. These two 

 vascular rings are connected by a supposed heart, which, according 

 to Ludwig, consists of a close plexus of contractile vessels. In the 

 Holothurians, besides the vascular ring round the oesophagus, only 

 two trunks with their branches to the intestine are known. The 

 blood is a clear, slightly coloured fluid, in which numerous colourless 

 blood corpuscles are suspended. 



Special organs of respiration are by no means universally found. 

 The entire surface of the external appendages, as well as the surface 

 of the organs suspended in the body cavity, and especially of the 

 intestine, appear to play a part in the exchange of the gases of the 

 blood. The sea- water very likely enters by the pores in the madre- 



