PHYSIOLOGY OF THE INVERTEBRATA. 35 



that there is a digestive cavity or alimentary canal with an 

 oral aperture, and usually an anus ; also a water-vascular 

 system, and a true vascular system. 



The Tkichoscolices. 



This class of the Annuloid Series is divided into the 

 following orders : — the Turhellaria, Eotifera, Trematoda, and 

 Cestoidea. 



(i) The Turbellaria possess a mouth or oral aperture, and 

 an alimentary canal,* but there is no anus, except in 

 the higher forms. The mouth is either at the anterior or 

 posterior end of the body, or sometimes it is situated in the 

 middle. The alimentary canal is lined by aa endoderm, and 

 between the endoderm and ectoderm there is a mesoderm 

 consisting of muscular and connective tissues. The ali- 

 mentary canal of the Turhellaria is either straight or 

 branched. 



(2) The Rotifera, or the " wheel-animalcules," have a 

 funnel-shaped oral aperture or mouth situated on one side, or 

 in the middle, of the trochal disc. The mouth of these 

 organisms is a great advance on all the forms previously 

 described. Its internal lining, as well as the trochal disc, are 

 abundantly provided with cilia, and at the posterior end of 

 this cavity there is a muscular pharynx (mastax) with an 

 armature consisting of four distinct pieces. The muscular 

 pharynx and its appendages are used in the mastication of 

 the prey seized by the ciliated trochal disc. The pharynx 

 leads into a short oesophagus (also provided with cilia) which 

 passes into a digestive cavity or stomach. The stomach then 

 passes into a short intestine. Opening into the anterior part 

 of the stomach are two large glandular tubes having a 

 pancreatic function. " The intestine, which usually opens 

 externally by a cloaca, has numerous lateral diverticula 



* With the exception of Convoluia, for in this form an alimentary canal 

 can hardly be said to exist. 



