112 



In embryos 16 — 17 hours old the larval mouth is about to form (Pl.XV, 

 Fig. 2, Fig. 14), in the usual place on the middle of the ventral side, the 

 embryo now being distinctly flattened. The entoderm, into which most 

 of the previously formed mesenchyme cells apparently merge, begins to 

 assume a somewhat comphcated structure (PI. XV, Figs. 7—13). While at 

 first a simple sac (PI. XV, Fig. 14) its lumen now gradually is subdivided 

 through a fold hanging down from the upper, thickened part of its wall. 

 It is not easy to get a full understanding of the real shape of the archen- 

 teron. But it is clear that there are two main parts, which represent the 

 hydrocoel and enterocoel pouches. There appears to be some variation in 

 the time of appearance of these structures. Thus in the specimen from 



which the figures 7 — 10 were drawn 

 the larval mouth had apparently not 

 been formed, while in the specimen 

 from which figures 11 — 13 were drawn 

 the larval mouth had developed, the 

 entoderm showing the same stage of 

 development as in the other specimen. 

 — As seen in PL XV, Fig. 13 there 

 is a distinct apical thickening of the 

 ectoderm at this stage. 



At the age of 18 — 20 hours the post- 

 oral arms are beginning to develop 



Fig. 46. Skeleton of the larva of Peronella (P^- XV, Fig. 3). The skeleton has 

 Lesueuri, 20 hours old. soo/^. al. anterolateral been formed (Fig. 40). It is Seen in 



this stage to be essentially of the 

 Clypeastroid type, but, as might be 

 expected from the rudimentary shape of the larva, it never develops into 

 the typical Pluteus-skeleton. In the later stages it forms a very irregular 

 network in the body, in which one can, with some difficulty, recognize 

 the body rod, the recurrent rod and the ventral transverse rod. (Comp. 

 fig. 2, p. 131, in Miss Medes' paper). It is noteworthy that an indication 

 of an anterolateral rod may be found in the young larva, although 'the 

 anterolateral arms are apparently never developed. 



In the young larva with the beginning postoral arms (PI. XV, Fig. 3) 

 there is a fairly conspicuous preoral lobe, with a distinct ectodermal 

 thickening at the point. This lobe, which is at first rather swollen, gradu- 

 ally diminishes in size and ultimately disappears completely, the mouth 

 opening lying then at the anterior end of the body, in the sinuation be- 

 tween the postoral arms; in the later stages the mouth opening even is 

 transplaced to the dorsal side (PI. XVI, Fig. 10). The normal shape of 



rod; b. body rod; po. postoral rod; r. recur 

 rent rod; vtr. ventral transverse rod. 



