the Sponges to the Corals. 211 



The canal-sjstem, with the circulatory and nutritive func- 

 tions dependent upon it, has, then, been demonstrated to obtain 

 in both the calcareous and siliceous sponges, as represented 

 by Grantia and Spongilla. Nor is evidence wanting to show 

 that the same arrangement holds good with the third order, or 

 Keratosa. 



Dr. Grant, in his interesting description of the excurrent 

 action of the sponges in general, remarks upon Spongia panicea 

 as exhibiting the strongest current he ever witnessed ; and, to 

 use his own words, he says, " Two entire round portions of 

 this sponge were placed together in a glass of sea- water, with 

 their orifices (oscula) opposite to each other, at the distance of 

 two inches ; they appeared to the naked eye like two living 

 batteries, and soon covered each other with feculent matter." 



The whole weight of Hackel's argument in favour of the 

 sponges being incorporated with the corals rests upon his in- 

 sisting on designating the excretory orifice of the sponge its 

 mouth or incurrent orifice, and in regarding the interstitial 

 canal-system as homologous with the coelenteric-vascular 

 system of the corals. Reflection alone, in connexion with the 

 foregoing facts, is sufficient to show his first assumption to be 

 both inconsistent and untenable ; and it is likewise a matter of 

 no great difficulty to demonstrate that his latter assumption of 

 homology of structure is entirely hypothetical. 



Now this coelenteric-vascular or gastrovascular system of 

 the Actinaria, what is it ? As may be shown, something far 

 simpler than the lengthy terminology made use of by Hackel 

 would seem to imply. 



A transverse section of any Actinozoon presents us with 

 the appearance shown at fig. 4 — a double tube, the inner one 

 of which, J, is the alimentary canal, and is brought into rela- 

 tionship by means of radiating connexions, the mesenteries, e, 

 with the outer one, or body-wall of the animal, g. This sec- 

 tion is supposed to be taken about halfway down in the region 

 marked h in fig. 3. The six spaces marked d, in fig. 4, are 

 the intermesenteric chambers ; and though separated from each 

 other by the mesenteries at this point of section, they com- 

 municate with each other freely lower down by means of the 

 common digestive cavity, fig. 3, c, of which, in fact, they 

 are simply prolongations. The region of the mesenteries, 

 surrounding as it does the alimentary cavity or canal, is 

 generally known as the perivisceral cavity ; into this all 

 the nutrient matters are passed, and undergo digestion, after 

 having traversed the alimentary canal of the animal ; and this 

 is what constitutes the coelenteric-vascular or gastrovascular 

 system of Hackel. Such is the essential and symmetrical 



