TERMINOLOGY. 



The following terms are usually employed in the description of the 

 species. A few additional terms, not of such general application, are also 

 used ; they will be found explained in their proper places. 



The test of a Sea-urchin is composed of the following parts. An apical 

 system, or, as it is also called, an abactinal system (PL IP.f. 1. e; III f. n), 

 composed of the anal system (PI. VII. f. is), the genital and the ocular plates; 

 the genital plates correspond to the summit of the interambulacral areas, 

 the ocular plates to the ambulacra! areas. One of the genital plates is 

 spongy, pierced with small holes, and is called the madreporic body (PL IP 

 f. i. <;). The anal system is not necessarily a part of the apical system (PL 

 XV. f 7). The coronal system, or test proper, is made up of ten zones, five 

 ambulacral and five interambulacral areas (PL VP.f. 1,2). The ambulacral 

 areas consist of a median zone (PL VII f. 4), carrying the tubercles, and 

 are either flanked by a poriferous zone or pierced for the passage of the suck- 

 ers. The pores are called unigeminal. bigeminal, trigeminal, when they are 

 arranged in single, double, or triple pairs of pores. When the pores form an 

 uninterrupted line from the apex to the mouth they are called simple ; they 

 are called petaloid when near the apex they expand and afterwards contract, 

 forming a leaf-like figure on the upper part of the coronal system (PL XP.f. 

 1). The interambulacral area consists of two vertical rows of plates extend- 

 ing from the apex to the mouth (PL VII f. 14, ie). The actinal system or 

 peristome is the opening closed by a membrane, in which the mouth (actino- 

 stome) is situated (PL IP.f. 2 ; PI VII. f. 20). 



The actinal system is, according to its shape and position, circular, central, 

 and notched, or eccentric, elliptical, transverse, and bilabiate. The poriferous 

 zone frequently assumes a peculiar shape round the actinostome, becoming 

 petaloid in some of the regular Echini (Pi. XIV 1 . f. 1) ; in others, where the 

 actinostome is pentagonal (Cassidulidae), the two poriferous zones are sep- 

 arated by the spreading of the ambulacral area, forming what is called a 

 fioscelle ; adjoining actinal ambulacral parts (called the phyllodes) are sep- 

 arated by an accumulation of small tubercles called bourrelets (PL XV. 

 f. d t 6), which correspond to the interambulacral region. 



