634 ECHINODERMATA 



her of small plates or scales, from which spines may project; a pair of 

 these called the radial shields, which are at the base of each arm, are 

 usually conspicuous. The plates surrounding the mouth form a complex 

 system (Fig. 983), the largest ones being the live buccal plates in the 

 interradii, one of which being also the madreporite. The five interradial 

 projections, which determine the form of the star-shaped mouth, are 

 called the jaws, and are composed each of a number of plates, from 

 which the teeth, tooth papillae, and oral papillae project into the mouth. 

 The arm contains typically four rows of superficial plates, an oral, an 

 aboral, and two lateral rows, and these surround an axial row, which 

 occupies almost the entire interior space of the arm; these plates are 

 joined together by muscle and connective tissue strands. The paired 

 ambulacral appendages pass between the ventral and the abutting lateral 

 plates in each arm. The axial plates originate as paired structures, 

 which unite more or less completely in the middle line. In a few forms 

 the arms are without definite superficial calcareous plates. The lateral 

 plates of the arms usually bear spines. 



The mouth leads through a short oesophagus to the sac-like stomach 

 which fills the disc. No liver sacs, rectum, or anus are present. The 

 ambulacral canals are intra-skeletal and consist of ring and radial 

 canals, the branch canals going to the ambulacral appendages, and the 

 stone canal going to the madreporite. Each radial canal ends in a ter- 

 minal tentacle; no ampullae are present. The ambulacral appendages 

 are not locomotory, but tactile, respiratory, and excretory organs, 

 and are called tentacles. The bursae take in and expel water, and prob- 

 ably aid in respiration and excretion. The entire nervous system is 

 intra-skeletal. 



The sexes are separate, with rare exceptions. The reproductive 

 organs consist of a large number of gonads in the interradial areas of 

 the disc, which open into the ten bursae. These pouches, which are 

 invaginations of the body wall, receive the genital products and extrude 

 them into the sea water; in Amphiura squamata and some other forms 

 they retain them while development goes on, so that in these cases the 

 young are born in the form of the adult. The gonads are all joined with 

 the axial organ in a manner similar to that in starfish. The axial organ, 

 which adjoins the stone canal, and is itself enclosed by the axial sinus, 

 sends a strand to the aboral side of the body cavity, which there forms a 

 ring. From this ring branches go to each group of gonads. The axial 

 sinus accompanies the ring and its branches and finally surrounds each 

 of the gonads. The larval brittle-star is called a pluteus. Brittle-stars 

 have great regenerative powers; a lost arm is replaced, and in many 

 cases the same is true of the upper surface of the disc, which may be 



