34 PHYSIOLOGY OF THE INVERTEBRATA. 



whole of the body (Grvegarina) ; by aay portion thereof 

 (Ehizopoda) ; by a mouth, an oesophagus, and a tolerably 

 definite portion of the sarcode of the body, without, however 

 (after the oesophagus), any distinct tube (Infusoria) ; by 

 an oral aperture or mouth, and a distinct alimentary canal, 

 one with a somatic or body cavity (Hydra) ; by an oral 

 aperture and a distinct alimentary canal suspended in, dis- 

 tinct from, but communicating with the somatic cavity 

 (Actinia). 



The Echinodekmata. 



In this class, physiological differentiation iu the digestive 

 apparatus has taken an important step forward. In the 

 Holothuridea there is a mouth and an oesophagus leading 

 into an alimentary canal, but it is not differentiated into a 

 stomach and intestine. The alimentary canal is simply a 

 tube with oral and aboral apertures. 



In the Asteridca, there is an oral aperture or mouth, and 

 an oesophagus leading into a wide stomach which has five 

 sacs round its periphery. The intestine is short and ter- 

 minates in an anus. In each ray there are two pyloric 

 cseca. 



The O'phmridca have a mouth, gastric sac or stomach 

 without caeca, and there is no intestine or anus. 



In the Echinidca there is a mouth provided with so-called 

 teeth (masticatory apparatus), and the intestine is long and 

 terminates in an anus. There is no differentiation into a 

 stomach and appendages. 



The Crinoidca* have an oral aperture which leads into a 

 short, wide oesophagus. There is a large, coiled, and saccu- 

 lated alimentary canal which terminates in an anus. 



In the Cystidca there is an aboral as well as an oral 

 aperture. 



Therefore, to summarise the Uchinodcrmafa, we may say 



* See Prof. Sars's papers, HJemoires povr sercir a la connais^ance cles 

 Crindidcs vivants, 1868. 



