STRUCTURE OF COMMON 8TAR-FI8H. ^T 



body over the mussel-beds, or rocks, or weeds, the arms 

 being capable of slightly bending ; after observing the red_ 

 eye-spot at the end of each arm or ray, and the movements 

 of the numerous spines wiiich are attached to the separate 

 plates forming the calcareous framework of the body- 

 walls, and examining the movements of certain modified 

 spines catted. pedicella/'im, whicli are pincer-lilce bodies situ- 

 ated among the spines, tlie student will be ready to study 

 the external and internal anatomy. 



First, as to the calcareous framework of the star-fish. 

 In order to study tliis, a transverse section should be made' 

 through an arm, and a vertical one through the body and 

 along the middle of a single arm, and finally the anim^il. 

 should be divided into two halves, an upper and lower. It- 

 will then be seen that the calcareous framework or so-called, 

 skeleton consists of a great number of limestone plates or 

 pieces attached by a tough membrane and covered by the 

 skin. Between the plates are spaces by which the v\ ater enters 

 the body-cavity tluough the skin. I'hese plates are arranged, 

 so as to give the greatest strength and liglitness to the body. 

 There is also to be seen an oral (actinal) side on which the 

 mouth is situated, and an aboral (abactinal) side, the re- 

 spectii'e limits of which areas vary greatly in the diifereut. 

 groups of Echinoderms. Each arm or ray is deeply chan- 

 nelled by the ambulacral furrow containing four rows of 

 suckers or "ambulacral feet," which are tentacle-like- 

 protrusions of the skin growing out through orifices in 

 the ambulacral plates, and are a continuation of the water- 

 sacs or " ampullse " within. The madreporic plate is a- 

 flattened hemispherical body situated on the disk between 

 two of the arms. It is perforated by canals. 



The nei'vous system of Echinoderms consists of a plexus of 

 cells and fibres overlying the surface of the shell. The oral 

 ring and radial nerves may be seen without dissection. By 

 closely examining the mouth, a pentagonal ring is seen sur- 

 rounding it, each angle slightly enlarging* and sending off 



* Owfsiannikoflf states that the nervous ring is a flat band, con- 

 taining no swellings or ganglia, and not differing in structure from th& 

 ambulacnil nerves, which latter possess nerve cells as well as fibres. 



