STARFISH 75 



careous spines, around the base of which numerous tiny pedi- 

 cellariae are grouped. Structurally and functionally the pedi- 

 cellariae resemble a pair of pincers, and they are able to seize and 

 to hold minute objects when contact is made. They function 

 primarily, it is believed, in keeping the body surface free from for- 

 eign materials which might be injurious to the delicate, respir- 

 atory branchiae. These are tiny filamentous projections of the 

 inner tissues which are found in considerable numbers in the areas 

 lying between the spines and the surrounding pedicellariae. The 

 central cavity of each branchia is continuous with the general body 

 cavity, and is filled with body fluid. The branchiae function in 

 respiration by the exchange of respiratory gases through their thin 

 walls. (W. f. 46.) 



Turning our attention to the central disc, a small, circular, per- 

 forated plate, the madreporite, is to be seen lying in the area of 

 the disc between two of the arms. The madreporite is the external 

 opening into the water vascular system, a unique feature of 

 Echinoderm structure which is described below. Near the center 

 of the disc is a minute anal opening which is practically non- 

 functional, as will be shown later. The two arms projecting from 

 the central disc on either side of the madreporite are designated 

 the bivium, while the other three arms constitute the trivium. 



Considering the Starfish from the under, or oral, surface, the 

 mouth will be found opening in the center of the disc, surrounded 

 by five groups of movable spines. Very prominent features are 

 the five ambulacral grooves which run from the mouth to the tip 

 of each arm. Throughout the entire length of each ambulacral 

 groove are numerous projecting muscular elements, the tube 

 feet, which constitute the chief structures of the water vascular 

 system for locomotion and the capture of food. 



Structure of the Body Wall. The body wall of the Starfish 

 contains many flat, articulating skeletal structures, the calcareous 

 plates, or ossicles, on which the projecting spines rest. Between 

 the ossicles, which in most regions of the body are irregularly 

 arranged, there are resistant connective tissue elements and muscle 

 fibers which form a common 'cement substance.' On the under 

 surface of the arms, the regularly arranged and specialized ossicles 

 form the walls of the ambulacral grooves in which the tube feet are 

 located. In the roof of the ambulacral grooves, the arrangement 

 of the double row of ambulacral ossicles is such that the opening of 



