24 OCEAN LIFE. 



cavity of the appendages into this or that bundle of them at will, 

 and so to alter the position of the centre of gravity of the blad- 

 der, and by thus bringing different regions of it to the surface, to 

 steer its course.] The appendages are of three kinds: Urticating, 

 digestive, and (probably) generative. The urticating tentacles 

 are the longest ; they are hollow, and are provided with muscular 

 fibres, of which the most conspicuous are longitudinal, and serve 

 to retract them. They contain many corpuscles of a reniform 

 shape, and are richly provided with thread-cells, whose filaments 

 are of the spiral kind. The gastric appendages are shorter and 

 wider, and are provided with stomata, which are applied to the 

 prey seized and benumbed by the tentacles. If the prey be small, 

 it is sucked bodily into the gastric sac ; if large, the sac becomes 

 distended with its juices and dissolved parts, the gastric secretion 

 being a very rapid and powerful solvent. The mouth of each sac 

 is wide, with a broad everted lip, armed with a series of i nettle- 

 cells.' The whole gastric appendage is highly contractile, and 

 in constant motion in the living animal. The appendages of the 

 third class are cyathiform." 



ECHINODERMA T A. (Asteroidea.) 



"THE casual observer who should, for the first time, examine a 

 star-fish, or a sea-urchin, two of the most familiar examples of 

 the Echinodermata met with upon our shores, would indeed find 

 it a difficult task to associate them with any other class, or to 

 imagine the affinities whereby they are related, either to the 

 simpler animals we have already described, or to more perfect 

 forms of existence hereafter to be mentioned. They would seem 

 to stand alone in the creation, without appearing to form any 

 portion of that series of development which we have hitherto been 

 able to trace. But this apparent want of conformity to the 

 general laws of development vanishes on more attentive examina- 

 tion ; so that we may not only trace the steps by which every 

 family of this extensive class merges insensibly into another, but 

 perceive that, at the two opposite points of the circle, the Echi- 

 dermata are intimately in relation with the Polyps on one hand, 



