414 WILLIAM J. DA KIN 



The development of these infra-cerebral vesicles is now 

 well known, l^liey arise as invaginations of the ectoderm (Text- 

 fig. 4) which is concerned in the formation of the supra- 

 oesophageal ganglia, and at first they are open to the exterior 

 (to the buccal cavity or near it), so that at this stage the supra- 

 oesophageal ganglia possess cavities which are open below. 

 The cavities become closed and then, whilst increase in size of 

 the ganglia takes place, they remain almost of the original size 

 (a slight decrease takes place if anything), and the surrounding 

 cells, which are indistinguishable from the ganglion cells, 

 become separated and pinched off from the brain mass, until 

 finally two small distinct vesicles lie appended as we have seen. 

 The diagrams illustrate how this takes place. 



One other feature of considerable importance remains to 

 be stated. In the adult one may occasionally find cells in the 

 infra-cerebral body undergoing mitotic division. This was first 

 described by Duboscq. We find, however, that the number is 

 much reduced as the animal becomes larger, and they are only 

 occasionally found in the full-sized specimens. In the small 

 Peripatus, not long born, they are more numerous (see Text- 

 fig. 3). It may be stated here that the same feature is to be met 

 with in the so-called ventral organs (Text -fig. 1). This fact 

 has not been recorded before and it completes the resemblance 

 between the -ventral organs and the infra-cerebral ve.sicles. 

 There can be little doubt of their homology. 



Discussion. 



The infra-cerebral vesicles of Peripatus were once con- 

 sidered to be sense organs concerned in hearing. Probably this 

 was by analogy with the little vesicles often found close to the 

 supra-oesophageal ganglia in the Polychaets and certain other 

 Invertebrata and once termed Otocysts. Thev are now usuallv 

 regarded as Statocysts or organs of orientation. Duboscq 

 concludes, however, that in Peripatus the}' are not sense 

 organs, nor glandular structures, but that in the adult as well 

 as in the embryo they are organs for the production of either 

 nerve-cells or neurogloea cells (supporting cells). 



