64 PHYSIOLOGY OF THE INl'ERTEBRATA. 



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

 caecal prolongations of the intestine, but in the higher 

 Cntstacca it becomes an organ of considerable size. As a 

 rule the salivary glands are better ditFerentiated in the 

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

 Arthro])oda. 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 Arthrojjoda the 

 alimentation considerably influences both the form and the 

 dimensions of the digestive apparatus. Carnivorous aniuials 

 have a digestive apparatus which is comparatively short. 

 Caterpillars, which are most voracious, have wide intestines, 

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

 have long and slender alimentary tubes. Certain genera of 

 the Inscda (JiJijhemera, Bonibyx, dec.) which are very voracious 

 as larvce, 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 Mcdacoscoliccs, one of the Malacozoic Series, is divided 

 into two great classes — the Puhjzoa and Brackiopoda ; 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 Hydrozoa. 



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

 genus Pihaldoph'uni. The disc or lophophore is horseshoe- 



