40 T. H. MORGAN. 



embryo give these points : The third pair of ganglia still have 

 large cavities and a small fourth pair is in process of formation. 

 The mid-gut sends the usual diverticula to the first and third 

 pairs of appendages, also quite a large pair of pouches to the 

 fourth appendages, and lastly a short pair, in the region of the 

 third ganglia, runs out at right angles to the gut. These last 

 pouches pass into that part of the embryo corresponding to the 

 sudden widening at the base of the middle posterior lobe. Sec- 

 tions show that at this place the ectoblast pushes into the body 

 cavity from behind forwards, forming partitions which are the 

 beginnings of a new pair — the fifth pair — of appendages. Pos- 

 teriorly the mid-gut seems to be continuous with a proctodeum 

 which, I think, forms at this stage. It also seems probable that 

 the heart forms at this stage of development, but the sections I 

 have do not speak definitely on this point. I could not discover 

 any trace, as yet, of reproductive organs. 



We come now to a stage which, I suppose on account of its 

 larger size, is easily found and not at all uncommon. I think it 

 is undoubtedly the stage formed immediately after the moulting 

 of stage 4. The small undeveloped fourth legs of stage 4 have 

 now, in stage 5, Fig. XY, spread out laterally after the shedding 

 of the cuticle, and have become segmented. The whole larva is 

 longer than in the last stage, and all the external organs are 

 also comparatively larger — the appendages, the proboscis, and 

 especially the posterior part of the body. The most marked 

 change is in the addition of the large fourth pair of appendages 

 — the first pair of walking legs of the adult. Each of these is at 

 this stage composed of six segments, of which the first three are 

 somewhat smaller than the following four distal segments. The 

 fourth segment of each limb, counting outwards from the body, 

 shows internally traces of division into two parts, although the 

 chitin covering the ectoblast is not divided. The spines pro- 

 jecting from the three distal segments of the legs must assist 

 greatly in clinging to the hydroids, and particularly the long 

 terminal hook with its two side hooks. Five pairs of ventral 

 ganglia are seen from the surface — the first, as we know, being 

 composed of two pairs. Other differences are easily gathered 

 from the figure. Sections show that the digestive tract opens 

 posteriorly to the exterior by means of a proctodeum. The first 



