86 CONNECTICUT GEOL. AND NAT. HIST. SURVEY. [BuU. 



firm, hemispherical shell made up of 20 meridional rows of closely 

 fitting plates, and with a wide circular aperture at one pole 

 (Plate XXV). In a living specimen of this same species (Plate 

 XX), the shell, or test, is so thickly covered with long stiff spines 

 as to obscure its surface completely except at the poles. 



At one pole of the body may be seen the five sharp, calcare- 

 ous teeth surrounding the mouth. This is therefore the oral, or 

 actinal pole of the body; the opposite pole being aboral, or 

 abactinal. Around the mouth is a broad area devoid of large 

 spines, called the peristone. It consists of a strong membra- 

 nous wall with an abundant musculature, enabling the mouth to 

 be opened and closed. The peristome is provided with numerous 

 slender pedicellariae, which differ from those of the starfish in 

 having three blades, which form the angles of a triangle. Sim- 

 ilar pedicellariae are scattered over the body. The circular lip 

 surrounding the teeth can close the mouth very tightly (Plate 

 XX). 



At the aboral pole there is likewise a considerable area (Plate 

 XX) devoid of spines, in the center of which is a cluster of 

 movable plates (Fig. 13) constituting the periproct. The intes- 

 tine opens in the center of the periproct. In the species under 

 consideration the periproct consists of four triangular plates, 

 while in the other native species of sea-urchin (Strongylocentro- 

 tus drobachiensis) there are many minute plates (Plate XXI). 

 The spines which cover the surface of the test vary somewhat 

 in shape, but all are cylindrical, solid, and longitudinally fluted. 

 These spines not only constitute a very formidable defensive cov- 

 ering, but are used in locomotion. Some of the spines nearest 

 the aboral pole are sharply pointed, while others are blunt ; below 

 the equator of the test the tips of the long spines are somewhat 

 flattened and rounded, while those nearest the peristome are 

 short, and spatula-like, with flattened, rounded tips (Plate XX). 



The base of each spine has a rounded socket which fits closely 

 against a corresponding hemispherical tubercle on one of the plates 

 of the test, allowing a great range of movement. Closely fitting 

 this ball-and-socket joint is a wreath of muscular fibers attached 

 both to test and spine. These muscles are able to move the spine 

 in all directions. Beneath these " moving " muscles is a wreath 

 of specialized muscle fibers whose function is, when stimulated. 



