66 EVERS'S COMPARATIVE ANATOMY. 



essential — including the alimentary tract; and the secondary, or 

 tributary, embracing the liver, spleen, pancreas, salivary, and 

 mucous glands. 



There is no organ so universally present, nor so essential, as an 

 internal digestive cavity, and hence so often alluded to as consti- 

 tuting a line of distinction between the animal and the vegetable 

 kingdoms ; indeed there is no class of animals without it, although 

 it is not found in every species of each class. The peculiarities 

 presented by the digestive apparatus will vary according to the 

 rank an animal holds in the scale of the creation, the kind of food 

 on which it is destined to subsist, and the elaboration that food 

 may require to undergo. The alimentary surface of a plant is the 

 exterior of its root spread out in the earth, and absorbing, by its 

 spongioles, the materials for its nutrition, but the alimentary surface 

 of an animal is internal, and continued from the skin, like the in- 

 side of a lady's muff. 



Cyclo-neura, Grant — Radiata, Cuvier. — In the very low 

 tribes of animals the digestive apparatus presents characters equally 

 diversified as the forms of the creatures themselves. The aliment- 

 ary cavity has often but one orifice; there are seldom teeth or 

 glandular organs, and the food is generally of the simplest sort. In 

 the polygastrica, as the term signifies, the cavities are numerous, 

 and their various forms have been well observed by Ehrenberg, 

 who caused these transparent animalcules to swallow fluids coloured 

 with carmine, indigo, or sap-green. The monads and several other 

 genera have but one orifice which receives the openings of the several 

 cavities, and from the absence of intestine the entire group has been 

 named anentera. in the higher forms, however, the alimentary 

 canal is furnished with an oral and an anal orifice, the straight, 

 curved, or spinal intestine receives the openings of numerous ccecal 

 appendices through its whole course. In some of the small monads 

 no internal cavity has been detected, and in other genera the num- 

 ber varies from one up to two hundred, as seen in a paramascium. 

 Many species are provided with dental organs in the form of stiff 

 spines disposed around the mouth, but in none of the class have 

 tributary or glandular organs been observed. 



The porifera present a very simple form of alimentary appa- 

 ratus, their absorbent canals closely resembling the ramified roots 

 of plants, and as we ascend through the diversified series of the 

 polypifera or zoophytes we perceive the system increasing in com- 

 plexity from the simple genito-digestive sac of the hydra up to the 

 complicated structure presented by the actiniform polypi, where the 

 stomach is provided with a muscular and mucous tunic lined with, 

 vibralile cilia, and communicates inferiorly with the genital cavities. 

 The greater number of the acalepha possess a simple broad and 

 radiated alimentary cavity, suited to their highly organised and 

 minutely divided food, but in the monostomatous species, which 

 live on coarser aliment, a cartilaginous masticatory apparatus and 

 biliary secretion are superadded. The torpid predaceous echino- 



