No. 19.] ECHINODEEMS OF CONNECTICUT. 69 



with a slender branch of the body cavity, together with small 

 nerves and branches of the water-vascular system. 



The mouth leads into a narrow esophagus, and this opens 

 into a large saccular stomach, which fills up most of the space 

 within the disk. There is no anal opening, the indigestible 

 remains of the food being cast out of the mouth. 



Disk. — The disk is covered with thin plates on all sides. 

 Certain of these plates present such definiteness in form and 

 arrangement that they afford the most convenient characters for 

 distinguishing the various species. 



On the oral surface five triangular groups of plates project 

 in toward the center of the disk and form the jaws (Plate XIII). 

 The jaws are usually provided with numerous toothUke processes 

 or flattened scales constituting the masticatory apparatus. These 

 processes are known as oral papillae, tooth papillae, and teeth, 

 according to the position they occupy on the jaw. The oral 

 papillse are toothlike projections along the edges of the 

 jaws, while the tooth papillae, when present, are similar structures 

 at the point of the jaw, and the teeth are five series of plates 

 which project from the mouth opening toward the interior of 

 the body. The oral papillae and teeth are well shown on Plate 

 XIII, while Fig. 9 represents in a diagrammatic way the rela- 

 tions of the same plates. The number and arrangement of these 

 processes are characteristic of each of the different species. 



The jaws are provided with a musculature which permits 

 them to be opened widely (Plate XIII and Fig. 9) or closed 

 tightly. In the specimen shown on Plate XIV two adjacent jaws 

 are thus closed, while the remaining angles of the mouth have 

 remained open. In most species the jaws, teeth, and oral papillae 

 are not so much used in crushing the food as in separating out 

 and swallowing the smaller or softer particles. 



A large plate at the base of each jaw is termed the oral shield, 

 while the pair of smaller plates on the face of the jaw, and 

 beside or internal to the oral shield, are the adoral plates (Plate 

 XIII and Fig. 9). 



On the aboral surface there is great variation in the number 

 and size of the plates, but one pair — the radial shields — lie at 

 the base of each arm. In some species, as Amphipholis squamata, 

 these radial shields are very large and conspicuous (Plate XVII), 



