ECHINOIDEA 291 



The Arbacia larva is very characteristic owing to its long, 

 obliquely posteriorly directed j^ostero-lateral arms (Fig. 165). 



Fig. 165.^ — Larva of Arbacia lixula. (After Job. Miiller, VII. Abhandl. 

 iiber Echinod. larven.) 



III. Suborder. Camarodonta Jackson. 



Test compact, never flexible. Ambulacral plates compound, 

 of the Echinoid type (Fig. 155, 2), the lowermost primary plate 

 being the largest. Primary spines wdthout an outer layer, 

 usually not very long, smooth, not hollow. Tubercles imper- 

 forate, crenulate or smooth. Teeth with a keel on the inside ; the 

 epiphyses of the dental pjTamids join above the teeth (Fig. 146, 1). 

 Usually 4 kinds of pedicellariae, globiferous, tridentate, tri- 

 phyllous, and ophicephalous. Spicules of tube-feet usually 

 regular, C-shaped. 



This suborder is divided into several families, three of which 

 are represented in the British seas, while a fourth, the Temno- 

 2)leuridce, may be expected to occur there also, though as yet not 

 known from there. 



Key to the families of Camarodonta known from or likely to be 

 represented in the British seas. 



1. Test usually with grooves in the horizontal sutures or on tho 



interambulacral plates, especially at the primary tubercles, 



which are crenulate or smooth . . Fam. Temnopleuridce 



Test without grooves ; tubercles smooth .... 2 



2. Gill-cuts at edge of peristome sharp and deep ; globiferous pedi- 



cellariae without side teeth on the valves ; usually with mucous 

 glands on the stalk and with dumbbell-shaped spicules in 

 the skin ..... II. Fam. Toxopneustidce 



