WATER SYSTEM. 695 



2, 8 ; PL XXXI. f. s) ; while the tentacles which are found near the actino- 

 stome are simple conical water tubes (PL XXX I. f. 14, 15), resembling the 

 water tubes which are found both in the ambulacral and interambulacral 

 spaces {PI. XXXI. f. 10, ll)\ they are connected with the ambulacral system 

 by long branches sent off from the main ambulacral tubes at right angles to 

 it (PI. XI f . f. 25), or scattered more irregularly over the abactinal side of the 

 test (PI. XI f . f. jo). These water tubes are usually confined to regular 

 lines on the actinal side, forming ambulacral furrows, which may be either 

 simple grooves, as in Clypeaster (PI. XP.f. 1) and Laganum (PL XIII'. 

 f. 7, 9), or may, as in Scutellirlae and Encopidae (PI. XIP.f. 2, 4 ; PL XIII d . 

 f. e), branch from the ambulacral over to the interambulacral space, and 

 sometimes extend, in Dendraster, almost to the abactinal system on the abac- 

 tinal side of the test (PL XIII". f. 3, 4). 



In the Clypeastroids we find modifications of the ambulacral regions 

 which are very characteristic of the suborder : the so-called ambulacral 

 fields covered by minute pores, which exist in their simplest form in Echino- 

 cyamus, Fibularia and Echinanthus ; these pores carry minute locomotive 

 tentacles, provided with suckers, strengthened by a calcareous ring or rod ; 

 they extend laterally in the ambulacral plates. In Clypeaster the minute 

 pores are most numerous along certain lines forming the central part of the 

 ambulacral zone, where they are closely crowded together without any 

 apparent order ; the smaller pores which carry the sucker-bearing tentacles 

 do not extend on the actinal side beyond the periphery. In another group 

 the furrows are either simple and straight, or fork at a short distance from 

 the actinostome, — the principal forks ramifying again more or less towards 

 the periphery ; these ramifications extend in several genera even into the 

 interambulacral spaces, — and it is in the lines of these furrows alone that 

 the minute ambulacral suckers are confined ; while in the Clypeastroids, in 

 which the pores extend over the surface of the ambulacral plates, they are 

 arranged either in regular horizontal rows parallel to the suture of the 

 plates, or without any apparent order. When seen from the inner side we 

 find in one section of the Clypeastroids that there are calcareous needles 

 running between these ambulacral pores ; in the other section these needles 

 are united to form a double floor extending over the whole inner surface 

 of the test, except over the petaloid part of the ambulacral zones, which is 

 only covered by small needles separating the rows of double pores. This 

 da^dalus of poriferous channels extends over the whole of the ambulacral 



