34 PHYSIOLOGY OF THE INVERTEBRATA. 



whole of the body (Gregarina) ; by auy portion thereof 

 {RMzopoda) \ 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 in 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 Asteridea, there is an oral aperture or mouth, and 

 an oesophagus leading into a wide stomach which has fiTS- 

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

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

 cseca. 



The Ophiuridea have a mouth, gastric sac or stomacli 

 without caeca, and there is no intestine or anus. 



In the Echinidea there is a mouth provided with so-caUeek* 

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

 terminates in an anus. There is no differentiation into &■ 

 stomach and appendages. 



The Crinoidea* 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 Gystidea there is an aboral as well as an oral 

 aperture. 



Therefore, to summarise the Uchinodermata, we may say 



* See Prof. Sars's papers, Mimoires pour servir d la connaissance def 

 Crinmdes vivants, 1868. 



