700 WATER SYSTEM. 



XXXII. f. s-io), though protruding from pores which have the regular 

 arrangement of phyUodes {PI XV. f. 1,8; PI XV a .f.2; Pi XXXII. f. 5; 

 PI. XXXIII f. 1). Hoffmann lias already called attention to the absence 

 of gills in Spatangoids ; he correctly suggests the presence of gills proper in 

 Cidaridae, although this had been denied by Midler. The cuts for their 

 passage are found, as 1 have already stated, not in the test, as in the regular 

 Echini (see PI. VII. f. 20), but near the actinostome in the actinal membrane; 

 so that their position is quite different in the Cidaridae (PL V. 6 ; PI. IP. 

 f. 2, .;; PL lP.f. 11), and in the other regular Echini (PI. I'.f. 6 ; PL IIP. 

 f. 8). 



In the Desmosticha the ambulacra) suckers, as is well known from the 

 early descriptions of Valentin and the subsequent memoirs of Stewart, are 

 strengthened by limestone plates (the disk) in the termination of the sucker 

 itself; the disk formed in the sucker consists of four terminal pentagonal 

 plates (PL XXXVIII.), the outer edge of which is more or less indented, 

 but the form of the marginal indentation docs nut enable us to recognize 

 with tolerable certainty the genus to which these disks belong, as the inden- 

 tations of the margin art' but so many points of growth which, with the 

 increase of the plate, become part of the interior of the disk, and at dif- 

 ferent stages present a very varied outline. These disks are riddled with 

 holes, formed by the component limestone rods. Immediately under the 

 terminal disk four smaller plates are placed, and they form a gradual pas- 

 sage, as far as I have been able to observe them in the Diadematidae and 

 Cidaridae. to the more simple ambulacra! calcareous spicules found along 

 the shaft of the tube (PL XXXVIII). The lower spicules appear to be 

 arranged without any special order, they are placed near the periphery of 

 the ambulacral tube, and are evidently of use in strengthening the thin 

 walls of the tentacles. In the Petalosticha (PI XXXVIII. f. :',. 25, ,>?), the 

 spicules in the terminal part of the actinal tentacles of the phyUodes pass 

 gradually into the irregular spicules of the shaft of the tube. The terminal 

 spicules differ in no way from those of the shaft, as there is no regular 

 disk composed of plates, but the thickened base of the long spicules (PL 

 XXXVIII. f. ,?4 -„'/;) is undoubtedly homologous to the plates of the suck- 

 ing-disk of Desmosticha. In Echinoneus (PL XXXVIII. f. 26) I have only 

 seen the spicules of the shaft, they remind us somewhat of the shorter 

 spicules of the Petalosticha ; there are no regular terminal plates as in the 

 Desmosticha, although the terminal sucking-disks are well developed in the 

 ambulacral tubes of Echinoneus. 



