ECHINOIDEA. 



215 



In the Spatangoids which have petals, the ambulacral plates 

 beyond the petaloid areas are large and the pores consequently 

 at some distance apart (Fig. 153 6). Moreover there is a tendency 

 in some species for the pores of a pair to coalesce into one. so 

 that some or all of them are or appear to be single. 



In the Clypeastroids there 

 are two kinds of pores : (1) 

 the yoked pores of the 

 petaloid areas which trans- 

 mit the large respiratory 

 feet, and (2) the minute 

 pores which are single, 

 though by. their elongated 

 form they may show signs 

 of being double, and which 

 transmit the locomotive feet 

 (see p. 233). The latter 

 alone are found on the lower 

 surface of the shell (Fig. 

 155) where they occur on 

 the interambulacral as well 



as On the ambulacral plates. of f he locomotive pores in the region of and 



beyond the petaloid part of the ambulacrum 



FIG. 154. Upper side of the anterior radius of 

 Clypeaster rangianus Desm., showing the ar- 

 rangement of the plates and the distribution 

 of the 



On the upper side of the (after j. M uHer). 



shell they are confined 



(almost entirely) to the ambulacral plates, and in the petaloid 



areas to those portions of the ambulacral plates which intervene 



between the two rows of yoked pores (Fig. 154). 



There are two modes of arrangement of the minute pores. In most 

 Clypeastroids (Clypeaster, Laganum, Arachnoides, Moulinia, Scutellina, 

 Echinocyamus, Fibularia, etc.) the pores and feet are distributed over the 

 whole surface of the ambulacra and in some genera extend, on the lower 

 surface of the shell, on to the interambulacra as well ; in such Clypeas- 

 troids the minute pores were described by Johan. Miiller as occurring in 

 pore-areas. In the Scutellidae, on the other hand, the minute pores 

 appear to be generally absent in the petaloid areas (except in Echinarach- 

 nius and Arachnoides) and on the lower surface of the shell are arranged 

 along grooves which may branch considerably and extend on to the 

 interambulacral plates (Fig. 157). Sometimes these grooves extend on 

 to the upper surface, but as a general rule they are confined to the lower 

 surface of the shell. These branching tracts of small pores were called 

 by Johannes Miiller, their discoverer, pore-fasciae. Branches of the 

 radial vessel follow them internally and are connected with the ampullae 

 of the tube-feet which issue from the pores. Similarly in the case of the 



