36 



Although this larva is thus comparatively well known, it does not 

 appear to me superfluous to give a full description and figures of its 

 different stages. 



Cultures were made in March— April 1916 at Tobago, B.W.I. The 

 fertihzation was undertaken on March 29th. After 6 hours there were 

 swimming blastulse. At the age of one day the first larval stage was reached 

 (PI. VIII Fig. 1)1). Its shape is the usual, the posterior end of the body 

 being short, truncated. Scattered redbrown pigment cells have begun to 

 appear. In regard to the skeleton it is very noteworthy that the body 

 skeleton does not form a basket structure. There is a recurrent 

 rod, but it does not unite at the posterior end with the body rod; it bends 

 inwards at about the middle of the body; below this horizontal part, 

 which represents the connecting rods (as I might term these rods connect- 

 ing the recurrent rods), it continues only as a very short, small process. 

 Both the ventral transverse and the connecting rods are pointed at the 

 end, and they do not join in the middle line as in the Tripneustes-lsLTva, 

 but transgress one another a little with their point. The body rod nor- 

 mally ends in two horizontal branches, one passing towards the ventral 

 midline, the other along the side of the body (as seen in fig. 4, p. 666 of 

 Tennent's "Variation in Echinoid Plutei"). The postoral rod is simple, 

 distinctly thorny, as is also the body rod. 



The postero-dorsal arms begin to form at the age of about 5 days; the 

 posterior transverse rod appears at the age of about 9 days, and at the 

 age of 12 days the larva has reached its full shape; the first pedicellaria 

 was formed in the 13 days old larva, and the first rudiments of the skeletal 

 plates of the young Echinoid were seen at the age of 15 days. — The 

 metamorphosis was not completed by any of the larvae, evidently be- 

 cause I transferred them at that time to another jar with a few algae for 

 the purpose of giving the young urchins good conditions; but they did 

 not stand the change and died. However, the main object was reached, 

 the development being traced to the full shape of the larva. 



The fully formed larva (PI. Ill, Figs. 1—2) has, like that of Tripneustes, 

 postero-lateral processes and vibratile lobes, and is, upon the whole, a 

 no less comphcate and beautiful object than the latter. The postero- 

 lateral processes are short, earshaped, and may be obUquely forward 

 directed. The posterior end of the body is convex, rounded, and the first 

 pedicellaria formed is not situated in a groove. The postoral arms are the 

 longest, of the same length as the body, while the postero-dorsal* arms 

 remain shorter, only about half that length. Possibly they may grow 



') This figure represents a 2 days old larva; it differs from that of one day only in the 

 arms bemg somewhat longer. ^ ^ 



