Chap. II. 



THE DIFFERENT RADIATA. 



61 



type, belonging to the order of Tabulata of Milne Edwards {Flys. 44, 45, and 40), 

 formed by animals entirely different from the true Actinoids, and closely allied, 

 as I shall show hereafter, to the genus Hydractinia, constituting a third type of 



Fi<]. 44. 



Fig. 45. 



Fig. 46. 



JIlLLEPORA ALCICOENIS, Llllk. 



A brunch of the Coral of that 

 name, natural size. The little pro- 

 jections along the edge are meant 

 for the extended Polyps. They are 

 extremely shy and delicate, and 

 never show themselves again after 

 a branch has once been taken out 

 of the water. 



MiLLErOKA AI.CICORNIS, Lmk. 



Magnified view of the extended 

 Polyps or Hydroids of the same 

 Coral stock. 

 a a Smaller Hydroids. — h Larger lly- 



drojd, 111 its mouth, t its tentacles. 



MiLLEPOEA ALCICORNIS, Lmk. 



Transverse section of a branch of 



the Coral stock, magnified. 



a a Pits of the Ilydroids, with their suc- 

 cessive floors. It is very difficult to 

 obtain sections of the pits occupied 

 by the smaller Hydroids. 



Coral stocks, which, on account of its Hydroid affinities, must be united with the 

 class of Acalephs. Moreover, these Corals differ greatly from those of the Actinoid 

 Polyps. The j^its into which the animals retreat have a horizontal floor extending 

 from wall to wall, and these floors are built successively one alcove the other, 

 as the animal rises, the radiating partitions never extending vertically through 

 successive floors. Not so with the Actinoid Polyps, in Avhich the radiating par- 

 titions extend from the top to the bottom of the pit, while the horizontal floors, 

 if they exist, extend only from one radiating partition to the other. 



Among Bryozoa we find a fourtli type of Corals. These Bryozoa are constructed 

 on a totally different plan, and exhibit a perfect bilateral symmetry ; for even the 

 whorl of feelers which surrovmds the mouth is not circular, but, like a horseshoe, 

 presents two symmetrical halves. From the mouth arises an alimentary canal, 

 extending in the longitudinal axis of the body, which Ijcnds itself in the same 

 plane, and, extending again forward, opens below the mouth. There is here no 

 sign of the characteristic partitions and chambers of the true Polyps, nor of the 

 radiating and circular tubes of the true Acale23hs: so that we need not even take 

 into consideration their bilateral structure, in order to satisfy ourselves that their 

 true position cannot be either with the Polyps or with the Acalephs ; while their 

 relation to the Ascidians and Brachiopods, and especially to the latter, is so close 

 as to place it beyond question now, that their true affinities are with the Mol- 

 lusks, and not with the Eadiates. 



I shall hereafter have an opportunity of showing that the comparative simplicity 

 of these animals is no evidence of any relation to the Polyps. The primary 

 question to be decided, in considering the true relations of animals, is not one of 



