ECHINODERMA TA. 1 73 



the aboral surface^ where those are absent, the skin is 

 strengthened by calcareous plates, armed with small 

 blunt spines and knobs of various sorts. The mouth 

 is in the middle of the lower surface upon which the 

 animal crawls, hence called the oral surface, while the 

 excretory aperture is on the tipper surface. 



The common Starfish {Asterias) is exceedingly de- 

 structive to oysters and other shell-fish. The Sun-star, 

 Solaster, has numerous rays, and is of a beautiful deep 

 crimson, varying to pink. The Brittle-stars [Ophiu- 

 ridw), which form a separate sub-class, have long, thin, 

 snake-like arms ; hence their name. They afford some 

 instances of a tendency to get rid of the larval stage ; 

 for while most of the Starfishes have a free larva, some 

 of them {AmpJdura) are viviparous, and the young do 

 not undergo metamorphosis. 



The Starfishes walk by means of their " tube-feet," 

 which are arranged in rows on the under side of the 

 rays ; each has a sucking' disc at the end of it. The 

 rows of tube-feet have received the name of ambulacra, 

 or walks : hence the hard structures which guard them 

 are called "ambulacral ossicles," and the groove down 

 each ray, on each side of which they are symmetrically 

 arranged, is called the " ambulacral groove " ; while 

 the water-vascular system is also spoken of as the 

 "ambulacral system." The way these feet are used is 

 as follows : first, a number of them are pushed out to 

 their full extent, then they catch on with their suckers, 

 contract, and so bring the animal on a little ; and the 

 same process is repeated. 



