OCULAR AND GENITAL I'l.ATKS. I ,- 



character, have a larger number of progressive variants as r,. K ar,N oeute plates than anv other 

 species of the genus (table, p. 162). 



Strongylocentrotus gibbosus is a most unusual sea-urchin in that it ha* deeply bedded in 

 the periproct a commensal crab, Fabia chilensis Dana. This mil, i, a v, , Mwebtoj 



but one specimen in 24 studied has no trace of a crab (text-fig. 155). Tl,,- Utomaloai MM 

 tion of the crab very deeply modifies the relative position of (! o,,,lar plate-. The 

 crab in the cases seen occupies a position close to ocular IV and genital : how., in the 

 figures, and no variation was noted in this position. This spe,-ie> :,,! (lyin.H-.-hi.Mi.- an- the 

 only Echini studied in which an aberrant arrangement of ocular plat,-, i. th,- dominant ,,i,,.. 

 In 24 specimens one has I, V, IV insert (text-fig. 154), and four hav,- all th.- .,,-nlar. fa 



TEXT-PIGS. 154-157. Ocular plate arrangement in Slrongylocmtrotux gibbosus (Agassi*). I':iy(:i. IVni. Tin- typir.-il 

 commensal crab, Fabia chilensis, in all but text-fig. 155. All figures X4. 



154. Diana. 31 mm. R. T. J. Coll., 704. Oculars I, V, IV insert. 



155. Diam. 28 mm. R. T. J. Coll., 702. Oculars V, IV insert. Crab wanting. 



150. Diam. 42 mm. R. T. J. Coll., 703. Oculars V, IV, III insert, the typical rhararlrr in this alx-rr.int np-. 

 the periproct meets the interambulacra at the points marked X. The commensal rnili is vrry rlrar and in it< lypir.-tl [op- 

 tion (pp. 94, 128, 177). 



157. Diam. 40 mm. R. T. J. Coll., 705. Ocular plates all insert. 



(text-fig. 157). These are the only cases of all oculars insert seen in the genus. It is also 

 interesting that in text-fig. 157 there is an interspace between ocular III and genital .'$. where 

 the periproct reaches the interambulacrum, and the same feature is shown in three arras in 

 fig. 156. This is comparable as a parallelism to a similar condition of separation of ocular and 

 genital plates in Phormosoma (p. 110). All the other specimens, 79 %, are alx>rrant in ocular 

 arrangement. In five, oculars V, IV are insert; one of these has no associated crab (text-fig. 



