By the Rev. T. A. Preston. 93 



tubercle (alluded to above) proceeds a tube down towards the oral 

 aperture. The tubercle is spongy and acts as a cullender, so that 

 the water, when passing through it, gets filtered, and all impurities 

 are prevented from entering the animal. The tube, on reaching the 

 oral pole, expands into a ring round the gullet, and from each point 

 of this ring opposite one of the ambulacral tracts springs a branch 

 passing along the middle of each tract. In addition to these five 

 branches are five other appendages, shaped like bladders, serving as 

 reservoirs for the supply of water. Thus there are the ring round 

 the mouth, the five bladder-like appendages, and the five branches 

 running up inside the corona. These five branches do not reach the 

 apical pole, but terminate shortly before reaching the apical aperture. 

 On their way they send out pairs of branches, one branch ending in 

 a bladder-like cavity, whilst the other branch divides into two, and 

 after passing through two of the pores, unites again and forms one of 

 the tubes of the suckers. These tubes, as mentioned before, are elastic, 

 consequently when the animal wishes to use one it forces some of 

 the water, which has entered by the madriporiform tubercle, into that 

 tube, till it is as long as necessary, and when it wishes to rest it, 

 it merely allows the water to flow back again, when the tube 

 contracts by its own elasticity. It may be mentioned that however 

 long the spines are, the tubes can always be made longer, even if 

 the spines are 3 or 4 inches long. 



There remains, finally, the dental apparatus, perhaps the most 

 marvellous arrangement, considering the otherwise low organisation 

 of the animal, to be found in the animal kingdom. 



I must first allude to the growth of the incisors, or front teeth, 

 of such animals as the rat, rabbit, &c. These teeth, being continually 

 used for gnawing, are — contrary to what occurs in other allied 

 animals — always growing. The tooth passes along the lower jaw 

 and terminates in a capsule which is continually supplying fresh ad- 

 ditions to the tooth, and thus the tooth is continually being pushed 

 forwards, consequently it is a necessity that the animal have some 

 means of wearing away its tooth at the other extremity ; and if from 

 any cause the animal is deprived of making a proper use of its tooth, 

 that tooth will grow on, and, being naturally curved, will grow round 



