268 
number of myotomes exceeds that of the gill-slits, but 
this is only a secondary phenomenon again. 
In Necturus Miss PLATT (1894, 1897) found a complete 
accordance of mesomerism and branchiomerism, the second 
gill-slit is situated beneath the auditory invagination, in 
front of the first meta-otic somite, the third one between 
the first and the second post-otic somite, etc. The same 
seems to hold for Gymnophiones, to judge from MARCUS'’ 
(1910) figures and description, though certain corrections in 
his interpretation of the somites seem necessary. Of the 
post-otic head-somites the last one still produces muscles 
of a permanent nature, while also the last but one still pro- 
duces a few fibres. GOODRICH (1911, p. 116), working on the 
axolotl, fully confirms Miss PLATT's conclusions regarding 
Necturus. 
If now in lower Vertebrates we find such an undeniable 
correspondence between branchio- and mesomerism, it is 
hard to deny or doubt such a correspondence in higher 
groups where the mesomerism in this region has become 
less distinct when the gill-slits appear. 
Paired intestinal dive. ticula among Protostomia. — For the 
solution of the problem of the relation of the branchio- 
merism to the mesomerism we look in vain to the Annelids. 
Neither here, nor in other Protostomia, do gill-slits occur 
and it need not be further argued, that there can be any 
question of a airect comparison to the gill-slits of Balano- 
glossus. Among the parenchymatous worms, however, we 
often find paired diverticula of the gut which remind us of 
the gill-pouches of Vertebrates. They otten alternate quite 
regularly with the equally paired genital follicles, from 
which MEYER (1890) derives the coelomic segments of 
Annelids. This is the case in several Turbellaria and in 
most Nemerteans. In Polyclads the ends of these diver- 
ticula sometimes apply themselves to the ectoderm, thus 
giving rise e. g. to the villi of forms like 7Thysanozoon, 
which no doubt have a respiratory function. In other forms 
again they may even open to the exterior (Yungia). 
Something quite similar is found among Molluscs in 
the Nudibranchs, which in several respects show such à 
remarkable resemblance to Polyclads. Here also we have 
paired diverticula of the gut, applying themselves to the 
ectoderm and giving rise to the gills or cerata, which often 
open to the exterior. In the year 1845 NORDMANN gave à 
