160 



BIOLOGY. 



[BOOK ii. 



form more strange still. In them has been established a division 

 into castes, as in our old human communities : but the caste in 

 question is a physiological caste. Certain members of the colony 

 specially adapt themselves for the digestive function. For that 

 purpose they come to bear the form of dilatable sacs, and are in 



communication interiorly with 

 the digestive cavity common 

 to all the tribe. 



In the medusae, and also the 

 actiniae, the digestive and cir- 

 culatory systems are still con- 

 founded, reducing itself to a 

 median cavity, whence star! 

 canals which unite on thj 

 edge of the ombrelle to fornv 

 a circular canal. (Figure 6.) J 

 The bryozoaries have a 

 mouth surrounded with ten- 

 tacles, an enlarged intestine, 

 sometimes furnished with 

 dentiform projections des- 

 tined to mastication. Occa^ 

 sionally there exists a sort of 

 stomach with orifices of inl 

 gress and egress (cardia and? 

 In the annelid we 



Fm. 



Half of aurelia aurita seen underneath, a, 

 marginal curpuscules ; t, marginal tentacles ; 

 b, buccal arms ; v, stomachal cavity ; gv, pylorus), 

 canals of the gastro-vascular system, which ** 7 

 ramify toward the edges and throw them- find a digestive tube nearly 

 selves into the circular canal ; ov, ovaries. 



complete with two orifices. 



The digestive tube of the lumbric has a powerful muscular 

 portion useful to an animal which feeds on the humus of the soil. 

 In the tunicates we meet with an oesophagus, a portion 

 enlarged, stomachal, and a rectum. The mouth is often at the 

 bottom of a large sac, whose walls serve at the same time for 

 respiration. It is another instance of physiological confounding. 

 We have mentioned above, tribes of animals bearing an intestine 



