160 



BIOLOGY. 



[Book ir. 



form more strange, still. In tli^m has been establisliefi a division 

 into castes, as in our old human communities : but the ca^te in 

 question is a physiological caste. Certain members of, the colony 

 specially adapt themselves for the digestive , Junction. Forth^t" 

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



communicatipn interiorly -with 

 the digestive cayity common 

 , to all the tribe. 



In the medusae, and also the 

 actiaise, the digestive and eir- 

 culatory systems are still con- 

 founded, reducing itself to a 

 median cavity, whence start 

 canals which unite on the 

 e,dge of the pmbrelle to form 

 a circular canal. (Figure 6.) 

 The brvozoaries have a 

 raputh surrounded wj;th ten- 

 tacles, an enlarged intestine, 

 sometimes furnished with" 

 dentiform projections des- 

 tined to mastication. Oocar 

 sipnally there exists a sort of 

 stomach with orifices of in- 



Fio. 6. 



Half of aurelifi aurita seen underneath, a, fri^ess and egresS (cardia and 

 marginal RornnsRules : t. niarsinal tentacles : "■ J . ,P , v ■, > i i,' 



marginal corpuscules ; t, marginal tentacles ; t j 



6, buccal arms ; v, stomachal cavity ; firu, pylorus). In the annelid ^ve 

 canals of the gastro-vascular system, vihich ■^ , ' ' i ' '' ' S 



ramify toward the edges and throw them- tiTi d a .digestive tube nearly 

 selves into the circular canal ; OTJ, ovaries. .'■"' ' '\ '''•^' ■'".":' '! ■' ''^iV'^ 



cpmnlp^te ^wim ,two ormces. 

 The , digestive tube of the , lumbric has a powerful muscular 

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

 In the tunicates we meet 'jyith an , oesophagus, a portim 

 enla,rg^d, stomachal, and a rectum. The moutK is pf ten at the 

 bottom of a laa-ge sac, whose wajls serve ^.t i^he same tim^ for. 

 respiration. It is another instance pf physiological confounding. 

 "We have mentioned above, tribes of animals bearing an intestine 



