OCULAR AND GENITAL PLATES. 



145 



character, have a larger number of progressive variants as regards ocular plates than any other 

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



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

 the periproct a commensal crab, Fabia chilensis Dana. This crab is a very constant associate; 

 but one specimen in 24 studied has no trace of a crab (text-fig. 155). The anomalous associa- 

 tion of the crab very deeply modifies the relative position of the ocular plates. The 

 crab in the cases seen occupies a position close to ocular IV and genital 3, as shown in the 

 figures, and no variation was noted in this position. This species and Gymnechinus are the 

 only Echini studied in which an aberrant arrangement of ocular plates is the dominant one. 

 In 24 specimens one has I, V., IV insert (text-fig. 154), and four have all the oculars insert 



756 



Text-figs. 154—157. — Oeiilar plate arrangement in Sirongylocenlrotiis giJihosus (Agassiz). Payta, Peni. 'I'lie tyjiical 

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



154. Diam. 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. 



156. Diam. 42 mm. R. T. J. Coll., 703. Oculars V, IV, III in.sert, the typical character in tliis aberrant species; 

 the periproct meets the interambulacra at the points marked X. The commensal crab is very clear and in its (ypical posi- 

 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 3, where 

 the periproct reaches the interambulacrum, and the same feature is shown in three areas in text- 

 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 aberrant in ocular 

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



