NO. 19.] ECHINODERMS OF CONNECTICUT. 21 



left. The two symmetrically placed rays adjacent to the madre- 

 poric plate constitute the bivium, while the three remaining rays 

 form the trivium. 



Body Walls. — The body walls, of both disk and arms, are 

 supported by a network of articulating calcareous plates, or os- 

 sicles (Plates II and VIII), held together by connective tissue and 

 muscular fibers. Imbedded in the body walls everywhere over 

 the surface of the body are blunt calcareous spines, arranged 

 in a more or less definite order, varying somewhat in different 

 species, and often movable upon the underlying plates. 



Covering the whole surface of the body, including even the 

 spines and pedicellariae, is a delicate membrane or skin, clothed 

 externally with closely placed vibratile cilia (Plate III). The 

 rapid vibration of these cilia keeps the sea water surrounding the 

 bodv in constant motion. 



Branchiae. — In the smooth area between the spines the body 

 wall is furnished with a multitude of protrusible filaments, the 

 branchiae (Plate I, fig. 4; Plate III), or respiratory processes, 

 which have very thin walls and are filled with the fluid of the 

 body cavity. When fully extended, an exchange of gases may 

 occur between the contained ccelomic fluid and the external sea 

 water, thus serving the purpose of respiration. 



Pedicellariae. — Scattered over the surface of the body in 

 many species of starfishes, and arranged in groups at the bases 

 of the spines (Plate III), occur great numbers of minute forceps- 

 like or scissors-like appendages, called pedicellariae. Each of 

 these organs is provided with strong muscles which enable it 

 to close tightly under the proper stimulus and thereby seize 

 any tiny object with which it may come in contact. In the smaller 

 pedicellariae the two blades cross like a pair of scissors, while 

 the others are like straight forceps (Fig. 8). By passing the 

 objects from one to another, the pedicellariae can by their united 



4 



Explanation of Plate III. Aboral surface of starfish, Asterias forbesi. 



Photograph of living animal. (Natural size.) Each of the large, 

 blunt spines is surrounded by a circle of tiny whitish pedicellariae, while 

 other pedicellariae and numerous minute tubular branchiae are scattered 

 irregularly over the dark colored skin between the spines. The madre- 

 poric plate is seen as a dark, circular area between the bases of the two 

 lower arms. 



