A COEAL AND A WOEM. 341 



a cylindrical cavity lying parallel to the main stem of the 

 Antipathes. Among the numerous specimens found by 

 Pourtalte not one occurred that had not a tube, and thus 

 the same thing has happened here as in the other instances 

 adduced. An abnormal peculiarity caused by a modification in 

 the mode of growth has, by the constant recurrence of the 

 exciting cause, become a distinguishing mark of the species. 



In conclusion I cannot refrain from mentioning one more 

 very singular and hitherto little-noticed case of the association 

 of two organisms which seems closely allied to the well-known 

 case of Lichens among plants. These, according to Sohwen- 

 dener's researches, are to be regarded as colonies of true one- 

 celled Algje and Fungi, and though individual botanists still 

 raise their protest against this view, the latest investigations on 



Fig. 90. — Antipathes fiUx, Pourtal6s, a homy coral of the West Indian seas, which by 

 constant association with an Annelid has been forced to form a tube for the worm. 

 Deep-sea corals. 



the subject seem to prove that they are no longer justified in 

 doing so. . 



The Sponges are now universally classed with animals ; 

 their soft parts consist exclusively of cells which are scarcely 

 ever co-ordinated to form special organs such as occur among 

 the higher animals. These soft portions are usually strength- 

 ened and supported by a network of fibres secreted from the cells 

 and extremely variable in structure. In the forms which are 

 generally regarded as the simplest and most typical, all the parts 

 unite to form a funnel attached by the pointed end, and of which 

 the free end has a large opening leading into a central cavity ; 

 this, for brevity, we will call the mouth of the sponge. But, 

 besides this, the internal cavity communicates with the sur- 

 rounding water by a system of fine canals which penetrate the 

 lateral portions of the sponge funnel in every direction. A 



