MUSEUM OF COMPARATIVE ZOOLOGY. 133 



figured by A. Agassiz* It is immature, and an important growth takes 

 place before it acquires the adult form. The pluteus which A. Agassiz 

 has figured is, according to my observations, about a week old. Eggs 

 artificially fertilized on July 16 developed into the pluteus, with the 

 antero lateral and the antero-internal arms just beginning to form, on 

 July 23. It was not easy to raise these plutei into older conditions, 

 but in the month of September there was fished from the surface of the 

 bay with the dip-net a complete series of plutei, which connects the 

 pluteus figured by A. Agassiz with the adult as here described, in which 

 all the four pairs of arms are of equal length. As the preceding plutei 

 were obtained by artificial fecundation, it is not to be supposed that the 

 fact that mature plutei are found in the middle of September, indicates 

 that these plutei are a month old. When artificially fertilized, the eggs, 

 however, were just ready to be laid. If, as A. Agassiz f says is the case 

 of Strongylocentrotus, the female Echinarachnius lays her eggs, or the 

 eggs can be fertilized at all seasons of the year, it would be very difficult 

 to determine the age at which the adult pluteus is attained from noma- 

 dic larvcC fished at random from the sea. 



A larval pluteus of Strongylocentrotus (Toxopneustes) fig. 52, f is 

 very similar to the stage of a pluteus of Echinarachnius at this age. 

 In Echinarachnius as in Strongylocentrotus, the antero-internal arms 

 are just beginning to appear, and although the antero-internal crescentic 

 spicules have already formed, the arms corresponding to these rods are 

 still quite small. This larva which was raised from the egg of Strongj-- 

 locentrotus is twenty-three days old according to A. Agassiz.t It would 

 thus be about two weeks older than my Echinarachnius of similar form, 

 also reared from the egg. 



The adult pluteus, PI. VII. figs. 1, 2, of Echinarachnius, first ap- 

 peared in great numbers at K'ewport in 1885, on September IG. In 

 former years they have been found earlier in the season. The older 

 stages were captured with a dip-net on the surface of the water, both by 

 night fishing and in the day-time. For a number of years I have kept 

 a record of the dates when our marine larvpe first appear in numbers, 

 and find that the adult pluteus of Echinarachnius is most common at 



* Revision of the Echini, p. 727. 



t Our common sea-urcliin (S. Drobachiensis) matures its genital organs in winter, 

 according to A. Agassiz. (Revision of tlie Ecliini, p. 709.) February is the month 

 wlien he onlinarily succeeded witli artificial fecundation. " Tlie sea-urchins spawn 

 during the wliole year." Op. cit. p. 719. 



t Op. cit. p 719. 



