SEA-URCHINS. 173 



The spines of the Petalosticha vary greatly in size, not only in different species, 

 but in the same individual . They are always delicate and silky, though in some 

 species they may be of great length. When the test is cleaned, its surface is in many 

 cases found to be marked by one or more symmetrical bands of close-set tubercles, 

 so small that a powerful lens is needed to distinguish them. During life these 

 tubercles bear slender spines, the heads of which are enlarged, while the shafts are 

 set with cilia, and shaft and head alike are covered with a thick skin. These fascicles 

 lie beneath or around the anus ; they surround the outer extremities of the petaloid 

 ambulacra, or, as in Amphidotics, they encircle the inner or apical terminations of the 

 ambulacra. 



Fascicles, as such, are recognized only among spatangoids, but it is probable, says 

 Prof. Agassiz, that the accumulation of miliary tubercles on the edge of some Phor- 

 mosomas must be regarded as the first trace of them. Thq earliest spatangoids, like 

 the Dysasteridae, have no fascioles. 



Throughout all these changes of position of mouth and anus, the genital and 

 ocular plates retain their central position ; but in some genera the genital orifices are 

 reduced to four. 



The circular ambulacral vessel in this order has no Polian vesicles, and there are 

 no vesicular appendages to the bases of the pedicels or suckers, of which there are four 

 kinds. These are single locomotive pedicels without any sucking disc; locomotive 

 pedicels containing a skeleton, and provided with terminal suckers ; tactile pedicels, 

 with papillose, expanded lips ; and triangular, flattened, more or less comb-like lamellae. 

 Two or three of these kinds of feet may occur in any ambulacrum, and those which 

 occur upon a fascicle are always different from the others. 



In the Cassidulid^ there are no fascioles, and the form of the test is sub-globular, 

 approaching that of the regular echini. It includes the sub-families Echineinae and 

 Nucleolinae. 



The mouth in this grouj) is placed centrally, or near the centre. Mhyncopygus 

 pacificus of the western coast of Mexico belongs here, as does also Catopygus recens, 

 which was found at a depth of one hundred and twenty-nine fathoms south of New 

 Guinea, and has an elevated test, the height about equal to the width, heart-shaped 

 when looked at from behind, and pointed in front. 



In the SPATANGiDiE, the principal family of the order, fascioles or bands of 

 crowded miliary spines occur, and a plastron, or space without large spines, surrounds 

 the oral opening, and is bordered by pores. This group is divided into several others, 

 the Pourtalesise, Ananchytinae, Spa- 

 tangina, Leskiinte, and Brissina, all 

 distinguished mainly by peculiarities 

 in the petals and fascioles. 



In Pourtalesia and its allies the 

 ambulacral system is simple, and the 

 plates which compose it are large. 

 The mouth, a large opening situated 

 in a groove, is elliptical, and is cov- 

 ered by a membrane strengthened by p,o 1^2.- PouriaUsia jeffreysu. 

 an outer row of plates. The species 



of PouHalesia have a curious snout, on the upper side of which the anal opening is situ- 

 ated. This snout gives to the test a most peculiar appearance when viewed from the 



