290 Proceedings of Royal Society of Edinburgh. [sess. 
in connection with the ectoderm, and in the form of a ring round 
the mouth (post-oral), and certain lateral cords. The coelome is 
simple, and the gonads are confined to the metacoeles, possibly to 
the left metacoele.* Here, as in the Ar chi-chorda, the gonads open 
directly to the exterior, and the metacoele no longer has paired 
nephridial openings. The vascular system when present consists 
of simple inter-coelomic spaces, and there appears to he a trace of 
the archi-coelomate central hlood-space, though there is some doubt 
as to the true nature of the organ. f As in Phoronis and Brachio - 
poda , so here the neo-embryonic stage of the free swimming larva 
( Dipleurula ) forms a culminating point in the elaboration of organs 
upon the bilateral archi-coelomate type. The larval forms will he 
referred to later, hut we need only note here that there is clear 
evidence of the penta-coelomic condition of the mesoderm. 
The anterior enterocoele or protocoele was first recognised, I 
believe, by Bury J as such, and the presence of two hydrocoeles or 
mesocoeles by Metschnikoff in certain larval forms, whilst the two 
metacoeles have always been easily identified. 
Bury,§ however, in the construction of his free ancestor of the 
Echinodermata does not appear to recognise the presence of a well- 
developed pre-oral lobe or protomere, and indicates the anterior 
enterocoele as no more prominent in this stage than in some of the 
present-day Echinodermata. Macbride|| has, however, by a happy 
comparison of the anterior enterocoele and hydrocoele with the 
proboscis-cavity and left collar-cavity of Balanoglossus respectively, 
shown to my mind the true relationships of the parts in the 
Dipleurula , and Bury’s anterior enterocoele then is clearly com- 
parable to the archi-coelomate protocoele, the hydrocoeles to the 
mesocoeles, and the two posterior enterocoeles to the metameres 
(fig. 5). As in so many sedentary Ar chi- coelomata, the protomere 
becomes vestigial, the mesomeres (or, more probably, only the left 
one) become much branched, and serve first as ingestive organs, 
and later, upon the re-acquirement of a free life, as locomotory 
* E. W. Macbride, Quart. Journ. Micro. Sci., vol. xxxiv. p. 149. 
+ H. Bury, Quart. Journ. Micro. Sci., vol. xxxviii. p. 129, and G. W. 
Field, Q. J. M. S., vol. xxxiv. 
X Q. J. M. S., App. 1889, and Phil. Trans. Boy. Soc. Lond., vol. clxxix. 
§ Q. J. M. S., vol. xxxviii. 
II Proc. Boy. Soc., vol. liv. 
