STAR-FISHES. 
105 
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 secs 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 ( Palinipes ), 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 ( Sculella ). 
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 flat pieces of definite geome- 
trical figures, some pentagonal, others hexagonal. And 
thus we have made our way to the curious flattened 
spheres which aro 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 wholo 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 ambulacral, pierced with minute pores 
