Chap. II. THE CLASSES OF RADIATA. 7I 



both ends. So also, in the dass of Acalephs. the digestive cavity in most Medusa} 

 is simply hollowed out of the central part of their spherosome, while in Ctenophoroe 

 that cavity has its walls, not only distinct from the spherosome, but in its upper 

 part these walls recede from the mass of the bod_y, and leave an open space 

 between the two, into which the products of dig-estion are poured. There is, 

 besides, in these animals, a double opening in the upper part of the spherosome, 

 through which the fajcal matters are discharged. Nothing of the kind exists in 

 any other Acaleph. Notwithstanding this, the Ctenophoi'a3 are strictly homologous 

 in all their parts to the other Acalephs. On the other hand, this peculiarity of 

 the digestive cavity of the CtenophorK! recalls the disposition already noticed in 

 some Star-fishes, and establishes a sort of transition between the extreme modi- 

 fications in the latter ; for, while the digestive sac of some Star-fishes is a closed 

 sac rising into the main cavity of the body witliout an open communication with 

 it, in other Star-fishes it rises to the upper wall of the l.)ody, through which it 

 passes, to open externally, and in Ctenophorte it opens into the cavity of the body, 

 the walls of which are in their turn pierced with two distinct openings, to afibrd 

 a passage for the fa3ces. These two openings cannot he considered as anal openings, 

 since they do not directly communicate with the digestive cavity; nor is the aljoral 

 end of the digestive sac to be compared to an anus, for it discharges its contents 

 directly into the main cavity of the ho(\y. We have here, throughout, combi- 

 nations which are entirely foreign to the plan of structure of the other branches 

 of the animal kingdom, and Avhich fully justify what I have already said above 

 respecting the impropriety of calling the parts of these animals by the same names 

 as those of other types. But while the Radiates are thus shown to differ in every 

 respect from the MoUusks, Articulates, and Vci'tebrates, they at the same time 

 become more and more intimately linked together, in proportion as we are better 

 acquainted with the typical features of their organization. 



As to the so-called external skeleton of Echinodemis, it in no way constitutes 

 a pecidiarity of this class, in contradistinction to the Acalephs and Polyps ; for in 

 Holothuriae the amount of calcareous deposits is comparatively small and does not 

 affect the flexibility of the spherosome, wliile the rigidity of the Echini is not 

 greater than that of the Corals compared to Actinia?. In both it is only a con- 

 solidation of the spherosome, resulting from the accumulation of limestone in its 

 tissue ; but the actinostome, as well as the diverticles of the aquiferous system, 

 the tentacles and ambulacral suckers, remains soft and movaljle. To judge correctly 

 of these relations, it is indispensable to observe these animals alive, with all their 

 soft parts fully expanded. In that condition Star-fishes and Sea-urchins have a very 

 different aspect from that which they exhibit when dried up or preserved in alcohol. 

 By comparisons made in this way we are enabled to establish the closest homology 



