STBVCTURE OF STAR-FlSn. 55 



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, 

 aod a vertical one through the body and along the middle of a single 

 arm, and finally the animal should be divided into two halves, an 

 upper and lower. It will then be seen that the calcareous frame- 

 work 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 small apertures by which the water 

 enters the body-cavity. These plates are arranged so as to give the 

 greatest streiigtli and lightoess to the body. There is also to be seen 

 an oral (under) side on wliich the moutli is situated, and an aboral 

 (upper) side. Each arm or ray is deeply channelled by the " ambu- 

 laoral furrow" containing four rows of suckers or "ambulacral feet,' 

 which are tentacle-like protrusions of the skin growing out through 



Fig 60 —Longitudinal section tlirough the body and one arm of Astenas vulgaris, 

 m mouth; s, stomach; i, lobe of stomach extending mto the arm; a, anus; 

 nr nervous ring; n, radial nerve; vr, water-vascular rmg, sending a radia 

 vessel (V) into the arm; mp, madreporic plate; t, stone canal; /i, hsmal 

 canal; oi>, oviduct; o, ovary; o)h, ampuUas, the ambulacral feet projecting 

 below'; b, cceca or liver. 



orifices in the ambulacral plates, aud are a continuation of the 

 water-sacs or "ampulla;" within. The " madreporic plate" is a flat- 

 tened hemispherical body situated on the disk between two of the 

 arms. It is perforated by canals. 



We are now ready to examine the internal organs and to study 

 their relations to one another and to the body-walls. The nervous 

 system may be seen without dissection. By closely examining the 

 mouth a pentagonal ring is seen surrounding it, each angle slightly 

 enlarging and sending off a nervous cord to the eye at the end of the 

 ray. It may be discovered by pressing apart the ambulacral feet 

 along the median line of each arm. Fine nerves are sent off to each 

 sucker, passing tlirough the opening between the calcareous plates 

 and extending to each ampulla, thus controlling the movements of 

 the suckers. The visible nerves belong to a nervous subcutaneous sheet. 



The month (Fig. 60, m) is capacious, opening by a short oesophagus 

 into a capacious stomach (Fig. 60, s) with tliiu disten.sible walls, aud 

 sending a long lobe or sac (Fig. 00, I) into the base of each arm ; 



