HIPPONOE ESCULENTA. 303 



spite of the apparent polyporous arrangement of the pores in three vertical 

 rows, we find, on analysis of the plates, that, as Dr. Lutken has shown, there 

 are but three pairs of pores for each ambulacra! plate. With advancing 

 age the arcs of these pairs of pores, at first quite distinct in young speci- 

 mens, and still to be seen in the very uppermost part of the poriferous zone, 

 gradually become more and more horizontal, and finally form three regu- 

 larly vertical rows, due simply to lateral crowding and not to additional 

 plates, as is readily seen in PL VIII. f. 29, the upper part of the poriferous 

 zone of a young Hipponoe' escnlenta, showing the gradual changes taking 

 place, from a vertical arrangement, as in Echinus, to the characteristic 

 poriferous zone of Hipponoe. 



Littoral to 10 fathoms. 



When alive the color of the spines of Hipponoe esculenta is white or 

 straw-colored, a darker color at the base, or brownish-yellow. The median 

 interambulacral space is spotted with black, the color of the heads of the 

 innumerable pedicellariae, scattered thickly over the whole of that part of 

 the test. The ambulacra! suckers are white at tip, gradually passing to yel- 

 low, and finally the basal part of sucker is of a dark-brown color. So that 

 when suckers are fully expanded, the tentacles form lighter bands, inter- 

 mediate between the black bands of the median interambulacral spaces. 

 The buccal membrane is reddish-brown as well as yellow, and the suckers 

 near the actinostome are of the same color. 



