12 TERMINOLOGY. 



two classes of columnar plates. The inter-ambulacral plates are all pentagonal, and 

 many times larger than the ambulacral; they carry on their external surface the large 

 primary tubercles. Of the five inter-ambulacral areas, one is single and posterior ; and in 

 all the Echinoidea which have the anal opening external to the apical disc, it is in this 

 single inter-ambulacrum that the vent terminates. The other four inter-ambulacra are dis- 

 posed in pairs, and form the greater part of the anterior and lateral parts of the test. 

 They are called anterior pairs and posterior pairs respectively, to distinguish them from 

 the odd area, which is called the single inter-ambulacrum. 



PI. I, figs. lb, 4 e ; PI. II, fig. 1 a, b, c, show the form and structure of the inter- 

 ambulacra in the Cidaridce ; and PI. VI, fig. 2 a, b ; PI. VIII, fig. 1 a, b, the structure of 

 these areas in the Echinidce. 



In the Cidaridce, and in some Echinidce, the inter-ambulacral areas have only two 

 rows of primary tubercles ; but in most of the Echinidce there are four, six, eight, or even 

 ten rows of primary tubercles in these areas. 



The Poriferous Zones. 



The poriferous zones are situated on each side of the ambulacral areas. (PI. I, fig. 4 a ; 

 PI. II, figs. 1 a, b, 2 b, c.) They are composed of a very great number of small pieces., 

 articulated together in such a manner as to form a series of holes, the corresponding edges 

 of the plates, remaining uncalcified at certain definite intervals, to produce foramina, which 

 are destined for the passage of retractile tubular suckers. The form and structure of the 

 poriferous zones afford a good generic character. As there are two poriferous zones 

 bordering each ambulacral area, it follows that there are ten zones. Some authors give the 

 collective name ambulacra to the zones and the area, but, for obvious reasons, we consider 

 them distinct sections of the test, and treat of them as such. The pores are round, oblong, 

 or elongated ; the pores forming a pair may be equal or unequal, or, in relation to each 

 other, they may be transverse or oblique ; they may be contiguous or remote, and when they 

 are united by a transverse sulcus, they are said to be conjugate. 



The pores are differently arranged in the zones in the different families. When they 

 are disposed in single pairs they are said to be unigeminal (PL II, fig. 1 c) ; when in 

 double pairs they are bigeminal (PI. VI, fig. 3 c, d) ; when in triple oblique pairs they are 

 trigeminal (PI. VI, fig. 2g; PL XII; PL XIV; PI. XV), or when they are grouped in a 

 greater number, as in many living species of the genus Echinus, they are polygeminal. 



When the zones extend in a straight uninterrupted line from the mouth to the apical 

 disc, they are said to be simple, as in the Cidaridce, Echinidce, Salenidce, Galeritidce, 

 Echinonidce ; when the zones, after parting from the apical disc, expand, and again con- 

 tract, thereby forming a leaf-like figure on the upper surface of the test, they are said to 

 be pelaloidcd, as in the Clgpeasteridce ; when the petal is not so complete as in the 

 Cassidulidte, it is sub-petaloidal. The zones are complete when they extend without inter- 



