64 PHYSIOLOGY OF THE INVERTEBRATA. 



glands. The " liver " in the lower Arthropods consists of 

 csecal prolongations of the intestine, but in the higheri 

 Crustacea it becomes an organ of considerable size. As a 

 rule the salivary glands are better differentiated in the 

 Insecta and Arachnida than in any of the other classes of the 

 Arthropoda. But the large bilobed liver, or, as we prefer to 

 call it, the pancreas, is characteristic of the Crustacea, 

 especially the higher forms. It appears that the salivary 

 glands and pancreas are interchangeable, sometimes one 

 replacing the other. 



It may be remarked that in many of the Arthropoda the 

 alimentation considerably influences both the form and the 

 dimensions of the digestive apparatus. Carnivorous animals 

 have a digestive apparatus which is comparatively short.- 

 Caterpillars, which are most voracious, have wide intestines, 

 while the butterflies, whicTi eat little, and only liquid foods, 

 have long and slender alimentary tubes. Certain genera of 

 the Insecta (JUphemera, Bombyx, &c.) which are very voracious 

 as larvEe, are in the mature state destitute of organs of mandu- 

 cation. Wholly destined for generation or reproduction, they 

 cannot take any nutriment ; hence the brief duration of their 

 lives. 



The Polyzoa. 



The Malacoscolices, one of the Malacozoic Series, is divided 

 into two great classes — the Polyzoa and Brachiopoda; and 

 the former class is subdivided into four orders. 



The Polyzoa have a mouth surrounded with tentacula, an 

 enlarged alimentary canal, sometimes furnished with denti- 

 form projections destined for mastication. Occasionally there 

 exists a sort of stomach. 



Most of the Polyzoa are microscopic animals ; but, living in 

 colonies, they sometimes form conspicuous masses ; conse- 

 quently they bear a resemblance to the Sertularian Hydrowa. 



(i) The Podostomata. — This order is represented by the 

 genus Bhabdopleura. The disc or lophophore is horseshoe- 



