212 THE MECHANICAL FUNCTIONS. 



Returning to the Asterias, we can trace changes 

 equally gradual, though in an opposite sense, in 

 another series, which presents a striking con- 

 trast with the former. Here, instead of the re- 

 trenchment of the appendages, we find them 

 greatly developed, and amplified in every pos- 

 sible degree. The rays of the Asterias become 

 narrower, while their length is at the same time 

 increased ; the vital organs, and also the tubular 

 feet, are gradually withdrawn from them, and re- 

 tire within a central disk, to which the slender 

 rays, now bereft of feet, become mere appen- 

 dages. Such is the condition of the Ophiura. 

 (Fig. 92.) By the prolongation and tapering of 

 these rays to slender filaments, they acquire a 

 greater prehensile power, and twine with ease 

 round their prey. We next find their number 

 augmented ; it is at first doubled, then tripled, 

 and at length indefinitely augmented. They 

 also become branched, subdividing by simple 

 bifurcations, as in the Euryale palmiferum (Fig. 

 93) ; next into minuter ramifications, as in the 

 Caput Medusa, where the thousands of filaments 

 have the appearance of a tangled web, which 

 defies all attempts at unravelling. 



The steps are but short from the Comatula to 

 the Crinoidea, or lily-shaped tribe, (of which, 

 Fig 94, representing the Pentacrinus europceus, is 

 an example); for they consist chiefly in the 



