STRUCTURE OF COMMON STAR-FISH. 97 



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 which are attached to the separate 

 plates forming the calcareous framework of the body- 

 walls, and examining the movements of certain modified 

 spines called pedicelhtria, which are pincer-like bodies situ- 

 ated among the spines, the 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 this, 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 anhrr.l 

 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 water enters 

 the body-cavity through the skin. These plates are arranged 

 so as to give the greatest strength and lightness 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- 

 specti^e limits of which areas vary greatly in the different- 

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

 nelled by the ambulacra! furrow containing four rows of 

 suckers or " ambulacra! feet," which are tentacle-like 

 protrusions of the skin growing out through orifices in 

 the ambulacra! plates, and are a continuation of the water- 

 sacs or " ampullae '" within. The madreporic plate is a 

 flattened hemispherical body situated on the disk between 

 two of the arms. It is perforated by canals. 



The nervous 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 



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

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

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



