Studies of Jamaica Echini. 145 



insert; 3 per cent are aberrant variants, all five specimens having oculars 

 I, V, IV, III insert. 



Considering the ocular-plate arrangement in Centrechinus from Montego 

 Bay as a whole, it is seen, as shown clearly in table 2 (page 156), that from 

 all exsert.the character of the young, there is a steady progressive series 

 with growth in the development or traveling in of ocular plates. Centre- 

 chinus is a member of the most primitive suborder, the Aulodonta, of the 

 Centrechinoida, and as such is worthy of special attention, for, as shown by 

 Professor Hyatt, primitive types in a group have a slow development as com- 

 pared with specialized types, and in character approach nearest to the next 

 lower series of their own phylum. The development of Centrechinus amply 

 bears out this important truth. 



Of the total 1,401 specimens of Centrechinus setosus listed from Mon- 

 tego Bay, the aberrants are 59 in number, of which 8 have ocular IV only 

 insert; 3 have oculars I, IV insert; 32 have oculars V, IV insert; i has 

 oculars I, V, II insert, and 15 have oculars I, V, IV, III insert. It is note- 

 worthy that in the mature series (50 to 80 mm. diameter) the only aber- 

 rants that occurred were cases of oculars I, V, IV, III insert, thus indicating 

 that the other aberrants seen in immature individuals may be considered 

 as irregularities in the order of sequence of the coming in of ocular plates, 

 rather than as aberrants that would have retained the given character if 

 they had lived to grow up. 



In specimens of Centrechinus up to 14 mm. in diameter (48 individuals), 

 all the genital plates are imperforate (fig. i), whereas in those 14.5 mm. and 

 all larger with very few exceptions, all genital plates have the genital pores.^ 

 In comparison with this, the specialized Strongylocentrotus drobachiensis 

 attains its genital pores when between 5 and 10 mm. in diameter (Phylogeny 

 of the Echini, pp. 131, 170). Centrechinus has all oculars exsert up to 15 

 mm. diameter, and with intermediate stages still 

 has 4 per cent all exsert when 40 to 50 mm. 

 diameter. Sfrongylocentrotus drobachiensis, on 

 the contrary, has typically oculars all exsert only 

 in very young specimens and has only 29 per 

 cent with all exsert in specimens (51) between 2.5 

 and 4 mm. in diameter; further, it has only 4 per 

 cent with oculars all exsert in specimens (82) 

 between 4 and 5 mm. diameter. Centrechinus 

 first attains the species character of oculars I, 

 V, IV insert as a dominant feature (45 per cent) 

 when it is 2 s to 30 mm. in diameter. Strongylo- ■*• 



,, ,. . ,,. T17-' i. ±.u^ Fig. I. — Centrechinus selosus (X^s^€). 



centrotus drobachiensis attams 1, V msert, tne Montego Bay, Jamaica, x 12. 



, \ • ^ c i. ^ f,.^ ^^.^ Diameter ii mm. OcuJara all 



species character, as a dominant leature (52 per exsert. No genital pores, large 



X , 'J- J- -. T« r'^^f^^ marginal plates in periproct. All 



cent) when 5 to 10 mm. in diameter. In Lentre- these are youthfiu characters. 



1 1 regret to say that by error in the figure published of a young Centrechinus (Phylogeny of th« Echini, 

 fig. 88. p 106), genital pores are shown. The specimen has no pores and they were madvertently drawn. 



