26 



The eggs are very small, only ca. 0.07 mm, and very clear. The cleavage 

 is of the usual, regular type, affording no special features. Swimming 

 blastulae were observed after about 16 hours; they are very clear and trans- 

 parent, with beautiful, polygonal cells, the limits being very distinct. 

 At the age of two clays the skeleton begins to form, and the Pluteus- 

 shape is assumed, but the arms are still very short. The three days old 

 larvae are typical young Plutei (PI. V, Fig. 5) with long postoral arms: 



there is a fairly pro- 

 minent red pigment 

 spot in the end of 

 these arms, and a 

 few red pigment cells 

 scattered on the 

 body. The suboral 

 cavity is very large, 

 reaching below the 

 level of the ventral 

 transverse rods. 



The skeleton (Figs. 

 12). The body 

 skeleton forms a 

 basket ^-structure, 

 very oblique; the 

 recurrent rods are 

 rather abruptly bent 

 at the point, where 

 the connecting rod 

 proceeds, the lower 

 part running from 



Fig. 1. Skeleton of the larva of Diadema antillarum: from the 



ventral side. 20 %. 



In all the figures of Echinoid larva; the same letters are used here very obliquely 

 to designate the same parts, al. anterolateral rod: h. body rod; (owards the noillt of 

 po. postoral rod; r. recurrent rod: vtr. ventral transverse rod. 



union with the body 



rod. The body rods are slightly curved: at the end they are widened 

 into a disk; also the connecting rods and the ventral transverse rods are 

 thus widened at the end, where they meet. Sometimes the disk may be 

 more or less irregular, so that they meet like a pair of clasping hands. 

 The whole skeleton is very little thorny, sometimes perfectly smooth. 

 The postoral rods are fenestrated. 



1 ) I have previously designated this type of structure of the body skeleton as a "frame": 

 the designation "basket" used by Tennenl seems to me more appropriate, and I there- 

 fore adopt it. 



