85 



plementary transverse rod, proceeding over UK- stomach, where it meets 

 the corresponding rod from the other side in the midline. The end of 

 this supplementary transverse rod again bifurcates, one branch going up- 

 wards, the other downwards; the latter branch is somewhat widened, 

 flat, with some few thorns along the edge, whereas the upwards directed 

 branch appears to be simple or at most carrying a single small thorn. 

 To this branch the ventral adductor muscle is attached in the species 

 e and f; it seems beyond doubt that there must also be such adductor 

 muscle attached to these rods in the present species, but on account of 

 its excessive contraction it is impossible to see it. The anlerolateral rods 

 are fairly robust, simple, or with a very few small thorns. .lust outside 

 the point of issue of the anterolateral rod a fairly long thorn proceeds 

 from the postoral rod. The recurrent rod is short, simple, with a small 

 medially directed branch at the end. The posterodorsal rod is fairly well 

 developed, almost as long as the anterolateral rod; it is simple and smooth : 

 at the base it is somewhat curved, with a well developed medially directed 

 process, the dorsal transverse rod, and another downwards directed pro- 

 cess. Its whole shape is somewhat unusual for a posterodorsal rod, recall- 

 ing to some degree an irregularly shaped dorsal arch; but I have hardly 

 any doubt that it is really the posterodorsal rod. especially because it 

 is a paired structure - - the dorsal arch not having made ils appearance 

 as yet. However, attention must be called to the fact that in species d, 

 which is evidently in the same slage of development, the dorsal arch is 

 well developed, while there are no posterodorsal rods. Full certainty re- 

 garding the true interpretation of these skeletal parts can hardly be acquir- 

 ed from the scanty material available at present. The posterior trans- 

 verse rod is bow-shaped, with a few small thorns along the posterior edge. 

 The ends of this rod appear to be simple, not widened or fenestrated 

 as in species e and f. 



Species d. One specimen was found in a plankton sample taken at 

 Taboga, Gulf of Panama, in January 1916. The stale of preservation is 

 not good enough to allow giving a full figure of it. especially because I he 

 preoral lobe is destroyed. But il is evident that this larva resembles the 

 species c very closely in the shape of the body. The posloral arms are 

 horizontally directed, thus giving the larva a very dilTerent aspect from 

 the specimen of species c shown in PI. XIII. Fig. 1; but this depends, as 

 stated above, only on the slate of contraction of Ihe muscles moving these 

 arms. The arms are broken, the remaining part being 7 times the body 

 length; that they must be really a good deal longer is evident bnl whether 

 as long as in species c cannot be ascertained. The aiiterolaleral arms are 



