GENEEA AND &I10TJPS OF THE ECHINOIDEA. 301 



oblique series, or to be oblique, when there is an outward slant of three or 

 more consecutive pairs from above downwards ; but it will be found, as a 

 rule, that the highest, and therefore innermost, of the three pairs is the 

 adoral pair of a compound plate, as in Pedina. In every pair the pore 

 which is nearest the interporiferous area is the adoral of the two, or 

 was so at an early age, and is or was in contact with the transverse suture 

 of the plate ; but during the growth of plates the pair may become removed 

 from the neighbourhood of the suture, and the adoral character lost. 

 Uniporous zones occur in parts of ambulacra and sometimes throughout. 

 Polyporous amholacra, as in Olypeastridse, are those in which besides the 

 usual pairs numerous single pores exist throughout or in part of the ambu- 

 lacrum. Pores are similar when both of a pair are the same in shape ; 

 dissimilar when the outer pores of a series of pairs differ in shape and 

 size from the inner series. Pores become obsolete or rudimentary/ or very 

 small in the zones between Internal Fascioles and the Apical system. 

 Pairs of pores may eitlier be flush with the test or open out in slightly 

 raised elliptical rims, called periiwdia ; there is usually a septum between 

 the pores of a pair, and there may be a costa between each pair of pores. 



Ambulacra are simple when the pairs of similar pores keep to slightly 

 curved lines of vertical direction, as in Cidaris, Biadema, and Echinocorys ; 

 and they are then often termed apetaloid in contradistinction to the 

 petaloid condition. Petaloid ambulacra are those which enlarge between 

 the apex and the ambitus, and contract again more or less perfectly 

 before reaching that region; they have large pairs of pores, and the outer 

 pores of pairs are usually broader than the inner, and this dissimilarity 

 increases with the boldness of the curvature of the zones. Subpetaloid 

 is a term given to ambulacra similar to those just noticed, but the pairs of 

 pores do not tend to close distally. The pairs of pores at the peristome 

 or close to it may be simply crowded ; or they may be in a pattern forming 

 a kind of petal, some of the pairs being voider apart than others, and some 

 put out of place and " doubled," this arrangement is a " Phyllode." When 

 accompanied by ornamentation of the tumid interradial peristomicd margins 

 (bourrelet), the whole forms a " Floscelle." 



The " Tentacles or Pedicels " of the ambulacra are placed over the 

 peripodia, and single, and may be prehensile and end in a disk, or be 

 branchicd or penicillate* ; they may be of the same kind or they may 

 differ in the same ambulacrum, especially if there are fascioles crossing 

 the areas, and then tlae tentacles of the peristomial region differ from 

 those of the dorsal. The tentacles of the ambulacra are homoiopodous 

 when they are similar, and heteropodous when they differ in shape, 

 construction, and function ; subheteropodous when the abactinal tentacles 

 are partly branchial and partly and feebly prehensile, the actinal being 

 with disks ; subhomoiopodous when the actinal tentacles, although modified, 

 still have suckers. Branchial incisions, grooves, or cuts are notches in 

 the peristomial margin for external branchiae, are on either side of each 

 ambulacrum in some orders of the Bndocyclica, and there may be a " Tag " 



* O. F. Miiller, 1789, 'Zool. Danica," vol. iii. p. 18. 



