126 



LECTURE VU. 



61 



Campanularia dichotoma, magnified. 



hydriform polype-animal, or association of polypes, resembles a minia- 

 ture tree ; but consists essentially of 

 a ramified tube of irritable animal 

 matter, y, defended by an external, 

 flexible, and frequentlyjointed, borny 

 skeleton, a ; and is fed by the activity 

 of the tentacula, d, and by the diges- 

 tive powers of the alimentary sacs, g, 

 of a hundred polypi, the common pro- 

 duce of which circulates through the 

 tubular cavities for the benefit of the 

 whole community. These currents of 

 the nutrient fluid have been observed 

 and described by Cavolini*, and more 

 recently by Mr, Lister.f The genera 

 Sertularia, Campanularia, Tubu- 

 laHa, &c., which form the principal 

 subjects of Ellis's beautiful and clas- 

 sical work on Corallines, compose the 

 present division of the compound Hi/- 

 drozoa, or hydriform polypes. The soft integument of the nutrient 

 polypes {fig- 61, d, e, g) is characterised by peculiar cells, like the 

 "dart-cells" of the Hydrae, which have a transparent firm cell-wall, 

 containing a clear fluid, and extremely delicate, sometimes spi- 

 rally disposed, filaments. These are protruded when the skin is 

 irritated, and give the tentacles the appearance of being beset by 

 bristled prominences. The digestive sac of each polype is lined by a 

 ciliated epithelium, and by hepatic cells, as in the Hydra : but its 

 base is perforated by the hole which communicates Avith the tube, f, 

 passing along the branch supporting the polype, to the general cavity 

 of the stem. But this outlet allows only the fluid contents of the 

 stomach to pass: the coarser rejectamenta are cast out by the mouth. 

 It appears that sea-water may have entry to these canals and cir- 

 culate with the chyle, and so contribute some share to the respir- 

 atory process of the corallines. It is certain that sea-water is admitted 

 to the corresponding cavities in the Anthozoa. Both Lister and 

 Lowen have observed an alternate imbibition and expulsion of water 

 in the polypes of SertuIaricB and Tubularice. The chylaqueous fluid, 

 as it may be termed, which circulates in the general ramified cavity 

 of the coralline is colourless, and contains only some minute round 

 corpuscles. This fluid is sent into the cavities of the prehensile arms. 



CVI. 



t CVII. p. 299. 



