174 PHYLUM ECHINODEEMATA. 



they are attached. Stalked pedicellariae are at the end of a short soft 

 stalk, the blades articulating with a calcareous piece at the end of the 

 stalk (Fig. 123, H and K, 2). Stalked pedicellariae are either straight 

 (forficiform) or crossed (forcipiform) (Fig. 123, H and K). In the forcipi- 

 form pedicellariae the blades cross at their lower ends. Three-bladed 

 pedicellariae constructed after the fashion of the straight variety are 

 occasionally found. The blades of pedicellariae may 

 be longer than they are broad, in which case they are 

 said to be forcipate ; or they may be broader than 

 they are long, in which case they are valvulate (Fig. 

 124). Valvulate pedicellariae are always sessile. 

 Alveolate pedicellariae are sessile pedicellariae in 

 124 v 1 it which the blades are inserted into a depression or 

 pediceilariae of alveolus in the calcareous plate. Stalked pedicel- 

 Perrfer) ^a omfof lariae are confined to the Brisingidae, Pedicellas- 

 the five pedicel- teridae, Heliasteridae, Asteriidae, Zoroasteridae, 



lariae shown in Stichasteridae, which are united together by Perrier 

 the figure. _ . , 



as Forcipulata. 



Spines are movable : those at the edges of the ambulacral grooves 

 can be bent over the groove. Pedicellariae are no doubt related to such 

 movable spines. However this may be, it is highly probable that 

 they are defensive weapons, seizing and damaging other organisms 

 which come in contact with them, and that they also serve for keeping 

 the skin clear of foreign growths (see the account of pedicellariae under 

 Echinoidea). 



The starfishes have some power of bending their arms and the 

 tips are generally turned upwards so as to expose the eye to 

 the light. 



Muscles of the body wall. Muscular fibres are found in the 

 inner parts of the dermis near the peritoneum on the dorsal 

 and lateral parts of the arm and disc. They are arranged in an 

 inner longitudinal and outer circular layer. Some of the fibres 

 of the circular layer pass outwards to be inserted into the cal- 

 careous plates. The longitudinal layer is specially thickened 

 on the abactinal side of the arms and disc to form a kind of 

 longitudinal muscular band. These bands unite at a central 

 point in the disc. 



Skeletal muscles. Special muscles passing between various 

 parts of the skeletal system are present. The most important 

 are : (1) Muscles of the spines and pedicellariae, (2) muscles 

 which pass between the two ambulacral ossicles of a pair, (3)' 

 muscles which connect the successive ambulacral ossicles. 



All three parts of the central nervous system are present, viz. 

 the superficial, the apical and the deep oral (Fig. 125 and p. 123). 

 The ectoneural tracts consist of a circumoral ring and of radial. 



