300 Proceedings of Royal Society of Edinburgh. [sess. 
In the Holochorda or Vertebrata the premandibular head- 
cavities, forming most of the eye-muscles, have already been 
compared by several writers * to the paired elements of the proto- 
coele, whilst the mandibular cavities are under suspicion as the 
Fig. 19.— Horizontal section of 
Amphioxus larva (diagram- 
matic). The identification 
of the mesocceles (2) is due to 
M ‘Bride. 
Fig. 20. — Horizontal section of 
Amphioxus embryo diagram- 
matic (after M'Bride) to show 
formation of mesoderm. 
collar-cavities, though further knowledge is required concerning 
their development. Their innervation (see below) agrees with 
this interpretation. 
In the Archi-chorda the archimeric segments are clearly defined 
* Cf. A. Willey, Amphioxus and the Ancestry of the Vertebrates, pp. 128- 
129, and 279. 
