154 Papers from the Marine Biological Laboratory at Tortiigas. 



are quite massive and are fused over the ambulacra in a long median 

 symphysis. They may attain a height of 14 mm. or more. The apophyses 

 are also relatively high and massive in large specimens. 



ECHINOMETRA LUCUNTER (Linn6). 



Close in to shore on the coral rock at Montego Bay and in very shallow- 

 water Echinometra lucunter (Linn6) occurs in abundance. Not only adults 

 but individuals of all sizes, down to very young, live in the same exposed 

 situation, where they get the full beat of the waves on shore. This species 

 clings tenaciously to the rock and has to be pulled off from the rock with 

 some effort. The ocular development of this species is very interesting, 

 but unfortunately this was not appreciated until it was too late to gather 

 more material and only a limited series was collected. These were supple- 

 mented by a few specimens collected at the same time by Mr. G. M. Gray. 

 A large series could easily be obtained in this locality. As Echinometra 

 lucunter is more or less elliptical in horizontal outline, being elongated in 

 the plane of the axis of interambulacrum 3 and ambulacrum I, the measure- 

 ments of sizes are given in length instead of in diameter, as in the other 

 species considered. 



Considering the developing series of Echinometra, a set of 34 specimens 

 measuring 4.5 to 15 mm. in length have all the oculars exsert. Of this 

 series, those from 4.5 to 10 mm. in length (26 specimens) were all wanting 

 in genital pores, but these pores exist in all the larger sizes examined. The 

 series 15 to 20 mm. in length, 23 specimens, has 91 per cent with all oculars 

 exsert and 9 per cent with ocular V insert. While ocular I is the first to 

 enter the periproct in the Echinidae and Strongylocentrotidae, it is ocular 

 V that typically enters the periproct first in the Arbaciidse and the Echino- 

 metridae, thus indicating family distinctions in ocular development. 



In the series 20 to 25 mm. in length, 58 specimens, in 71 per cent oculars 

 are all exsert and 29 per cent have ocular V insert. In the 25 to 30 mm. 

 series, no specimens, 34 per cent have oculars all exsert; 2 per cent have 

 ocular I insert. These are the only cases with ocular I only insert found in 

 Montego Bay material and it is a rare variation in the genus and family, as 

 previously shown. In 54 per cent ocular V is insert. This series is the first 

 stage in growth in which the typical species character of V insert is a dom- 

 inant feature. In 10 per cent, oculars V, I are insert and in the limited 

 number of specimens this is the first stage at which this character appeared. 

 With more material some specimens would probably be found having I, V 

 insert at an earlier stage of growth. This series is considered the last of 

 the developing stages, but the line as regards ocular development is arbitrary 

 because, as seen in the later series, oculars continue to come in with increas- 

 ing size of the individuals. 



Of the series 30 to 40 mm. in length, 175 specimens, 17 per cent have 

 oculars all exsert as arrested variants, 58 per cent have ocular V alone 

 insert as the typical species character, and 25 per cent are progressive 



