196 



METAZOAN PHYLA 



spines, known as marginal spines. These project more or less over the 

 groove, which is nearly filled \\'ith tube feet. 



225. Skeleton and Musculature. — The skeleton of the starfish con- 

 sists of a large number of calcareous ossicles bound together by connec- 

 tive tissue. These ossicles are regularly arranged about the mouth and 

 in the ambulacral groove, where they form flat plates lying vertically 



Per/'brar/^ch/'a/ space 



Pen'-f-oneum 

 Coe/om 



Tube /bof 



Pcrd/a/ 

 nerve 



Spi'tre 



Py/or/c cctecufTT 

 Derwobranchia 

 Ped/'cel/ana 



Ampu//a 

 Paolia/ ca/^a/ 

 Lafera/ carrcif 



'Ambu/acral p/a-^e 

 ^' ^ ^A/fcrrgi/'/^a/ sp/ne 



Ambu/crcro'/ 

 pores 



Ambcz/acra/^ A(pfa/77bu/cfcra/ 



p/a-^e g p/ai^e 



Fig. 110. — Ray of a starfish. A, diagrammatic cross section to show especially the 

 arrangement of the ambulacral plates and the tube feet. 5, ambulacral and adambu- 

 lacral plates of a dried and cleaned specimen, showing the pores for the tube feet. X 3. 



face to face on either side of the mid-ambulacral line and nearly at right 

 angles to it (Fig. 110 5). The plates in the ambulacral groove are also 

 inclined, so as to give the groove the form of a trough when looked at 

 from below. These ossicles, the fibrous tissue between them, and the 

 spines, as well as the pedicellariae, are covered with a soft epidermis. 



The rays of the starfish are not rigid but may be moved about by 

 muscle fibers in the body wall. There are also longitudinal and trans- 

 verse muscle fibers in the ambulacral groove which make it possible for 

 the rays to be bent toward the aboral side and for the ambulacral groove 

 to be closed by the drawing inward of the margins. 



