158 Papers from the Marine Biological Laboratory at Tortugas. 



sidering specimens more than half-grown as being fully developed in this 

 respect. This view was based largely on studies of Strongylocentrotus dro- 

 bachiensis, and in this species the view was fully justified. It is doubtless 

 true of many species, perhaps most species of Echini, that specimens half- 

 grown have practically the full character as regards ocular development. 

 It is not entirely true, however, of all species, for as shown in this paper, 

 in Centrechinus setosus, Tripneustes esculentus, and Echinometra lucunter 

 (table 2), ocular plates continue to come in or enter the periproct as a 

 developing character until nearly or quite grown; therefore, as a result in 

 these species, the largest numbers in percentage of oculars insert may be 

 found in the series of largest specimens. This is not true of selected indi- 

 viduals. That is, the largest specimen does not necessarily have more 

 oculars insert than a smaller individual, but taking a large series of indi- 

 viduals of these species, as shown in the table, the average of each size up 

 to the largest has more oculars insert than any of the preceding sizes. This 

 late coming in of oculars in some species is of interest as showing the mobility 

 in adjustment of the test to allow these changes to take place. It is also 

 of interest in support of the view urged so much by Professor Hyatt, that 

 changes are taking place constantly throughout the life of the individual. 



Centrechinus setosus is slow and very gradual in developing its characters. 

 It is slow in acquiring genital pores and very slow relatively in having 

 ocular plates enter the periproct. Up to 15 mm. in diameter no oculars 

 have yet entered the periproct. In the series 20 to 25 mm. in diameter, a 

 little more than half of the specimens still have all oculars exsert. When 

 35 to 40 mm. in diameter, which is about half-grown for the locality, the 

 species character of I, V, IV insert is developed to a high point, 71 per cent, 

 practically the maximum percentage; but at this age there are more speci- 

 mens representing developing characters and fewer representing progressive 

 characters than in older series. The maximum of the species character and 

 also the maximum of the progressive characters of I, V, IV, II, and all oculars 

 insert, is seen in the largest specimens of 50 to 80 mm. diameter. The 

 slow rate of development of Centrechinus as here shown (p. 156) is graphically 

 brought out by comparing it with that of the development of Strongylo- 

 centrotus drobachiensis given in the " Phylogeny of the Echini," page 142. 

 Centrechinus is a relatively primitive (and geologically old) genus in the 

 lowest suborder (the Aulodonta) of the Centrechinoida. On the other 

 hand, S. drobachiensis is a highly accelerated species in the highest suborder 

 (the Camarodonta) of the Centrechinoida. As Professor Hyatt showed 

 fully in his studies of fossil Cephalopoda, primitive genera have a slow 

 development and specialized genera have a highly accelerated development. 

 I have shown the same thing in Pelecypoda where the primitive Avicula and 

 Pecten have a relatively slow development, whereas the specialized Spondylus 

 and Ostrea have an accelerated development. 



Another feature of note in the ocular arrangement of Centrechinus setosus 

 is the fact that while ocular I or ocular V may be the first to enter the peri- 



