STAR- FISHES. 105 



r 



ous fluid, secreted and poured out from the lobes of the 

 stomach. 



The order which includes the Star-fish is very exten- 

 sive, comprising many species, and even many genera. In 

 a series of these genera, such as any well-stocked museum 

 affords, the naturalist sees a gradual deterioration and ob- 

 literation of the rays, and a commensurate development 

 of the disk or body. This double change proceeds by the 

 filling up of the angles between the arms, until the out- 

 line, instead of being five-rayed, is five-sided. A beautiful 

 British species, the Bird's-foot Star (Palmipes), affords an 

 example of this pentagonal form; 



From this condition it is easy to imagine the disappear- 

 ance in other species of the very angles themselves ; the 

 sides become progressively convex in their outline, and at 

 length a figure nearly orbicular is attained. Such, in 

 short, is the aspect of one of the rarest of British Echino- 

 dermata, the Cake- urchin (Scutella). 



The integument by this time has changed as well as the 

 form, having become shelly, presenting a hollow box, built 

 up of many thin and nearly fiat pieces of definite geome- 

 trical figures, some pentagonal, others hexagonal. And 

 thus we have made our way to the curious flattened 

 spheres which are characteristic of the Sea-urchins. Many 

 of the links which perfect the chain are, it is true, exotic 

 species : but even in British forms it is not difficult to 

 trace the connected progress from type to type — one of the 

 most beautiful gradations in the whole circle of Zoology. 



The shelly case of an Echinus is indeed an exquisite 

 structure. It is made up of twenty rows of plates, of 

 which five pairs are ambukcral, pierced with minute pores 



