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 area 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 larvee fished at random from the sea. 
A larval pluteus of Strongylocentrotus (Toxopneustes) fig. 52, 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 Strongy- 
locentrotus is twenty-three days old according to A. Agassiz.¢ It would 
thus be about two weeks older than my Echinarachnius of similar form, 
also reared from the egg. 
The adult pluteus, Pl. VII. figs. 1, 2, of Echinarachnius, first ap- 
peared in great numbers at Newport in 1885, on September 16. 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 larve first appear in numbers, 
and find that the adult pluteus of Echinarachnius is most common at 
* Revision of the Echini, p. 727. 
+ Our common sea-urchin (S. Drébachiensis) matures its genital organs in winter, 
according to A. Agassiz. (Revision of the Echini, p.709.) February is the month 
when he ordinarily succeeded with artificial fecundation. ‘“ The sea-urchins spawn 
during the whole year.” Op. cit. p. 719. 
t Op. cit. p 719. 
