33 



course of 24 hours; in my own experiments I did not fmd the young 

 Plutei until about two days after fertiUzation. 



As described by Tennent this first larval stage (PI. VIII, Fig. 2) has 

 the posterior end of the body truncated, the body skeleton forming a very 

 regular basket-structure. The skeleton (Fig. 6) is very smooth, the body 

 rods and ventral transverse rods, as well as the rods connecting the re- 

 current rods are slightly widened at the end, where they join. There is 

 a pronounced and characteristic sinuation at the point of issue of the 

 postoral rod from the body rod. The postoral rod, which is gracefully 

 curved at the base, is fenestrated; but the holes are small, excepting the 

 lowermost one, and gradually disappear towards the point. Red pig- 

 ment is beginning to appear in the point of the postoral arms, and also 

 some few scattered red pigment cells are found in the body. 



In this stage the larvae remain essentially unchanged for an unusually 

 long time. It was not until the 12th day after fertilization that the form- 

 ation of the posterodorsal arms began. That this long duration of the 

 first larval stage is a normal feature can hardly be doubted, since the 

 larvae afterwards developed quite normally. Also two other cultures gave 

 similar results, though these latter were not kept so long a time that 

 the beginning of the second stage was reached. 



At the age of 17 days the body skeleton had begun to be absorbed, 

 and the larva was about to assume its full shape (PI. II, Fig. 1); the vibratile 

 band had reached the hindend of the body on the sides, being drawn out 

 into a pair of lobes, the posterolateral processes. The posterior trans- 

 verse rod was found to appear on the 20th day. 



The fully formed larva (PI. II, Fig. 2), 22 days old, is a strikingly beautiful 

 object. The arms are all broad, especially the antero-lateral arm is 

 broadly widened at its base; the postoral and the postero-dorsal arms are 

 slightly and gracefully bent. The arms are all rather short, the longest, 

 the postoral ones, only slightly exceeding the length of the body. The 

 postero-dorsal arms are somewhat shorter, and the preoral and antero- 

 lateral arms are generally quite short, though they vary somewhat in 

 length. The postero-lateral processes remain short, ear-shaped lobes; 

 above these lobes the body is distinctly narrowed, which feature, together 

 with the concave outline of the posterior end of the body, gives the fully 

 formed larva a very different appearance from that at the beginning 

 of the transformation from the first to the second larval stage (Comp. 

 PI. II, Figs. 1 and Fig. 2). There are two pairs of vibratile lobes (auri- 

 cules); the dorsal ones, at the base of the postero-dorsal arms, are out- 

 wards directed, the ventral ones, between the base of the postoral arms 

 and the anal lobe, are forwards directed. The anal lobe is deeply concave 



