t^6 PHYSIOLOGY OF THE INVERTEBRATA. 



the middle portion of the stomach (which is sometimes called 

 a gizzard), and, aftei* having undergone a further comminu- 

 tion, are returned to the anterior portion of that organ, where 

 they are kept in constant agitation. These particles finally 

 pass into the intestine. The undigested portion of the food 

 agglomerates into small pellets which are carried upwards 

 and are expelled through the anus. 



The alimentary canal of the Folyzoa is devoid of salivary 

 glands. 



It has been stated that the Polyzoa resemble somewhat the 

 Sertularian Hydrozoa, but it must be distinctly understood 

 that the Polyzoon has not merely a digestive cavity, like the 

 Hydra and the Actinia ; for the digestive apparatus of the 

 former is differentiated into a pharynx, stomach, and intestine 

 provided with an anal aperture. In fact, the Polyzoon has a 

 •complex and highly developed digestive system. Then, again, 

 the Polyzoan tentacula differ from those of the Hydrozoa and 

 Actinozoa, in being somewhat stiff and provided with cilia. 



(3) The Gymnolccmata are marine Folyzoa, except Fahidi- 

 cella, which is a fresh-water form. 



(4) The Fcdicdlinca. — In this order the buds, produced by 

 gemmation, become detached from the original stock. 



The Brachiopoda. 



This class is divided into two orders, the Trdentcrata and 

 the Clistcnterata. They are all marine animals " provided 

 •with a bivalve shell, and are usually fixed by a peduncle, 

 which passes between the two valves in the centre of the 

 hinge line, or the region which answers to it in those Brachi- 

 •opods which have no proper hinge." 



(i) The Trdentcrata have no hinge. The mouth or oral 

 aperture leads into an oesophagus, which passes into a stomach 

 provided with pancreatic follicles. From the stomach passes 

 the intestine, which opens into the cavity of the pallium or 

 mantle on the right side of the mouth. The alimentary canal 



