INFRA-CEREBRAL ORGANS OF PERIPATUS 415 



The vesicles known as statocysts or otocysts in the Inverte- 

 brates are still of questionable function in many cases. This is 

 particularly so in the case of certain Nemertines (Metanemer- 

 tines) (3), where the walls of the vesicles are surrounded by the 

 ganglion cells of the brain mass and no cilia are present. In 

 fact they are not unlike the infra-cerebral vesicles of Peripatus. 

 On the other hand, in Molluscs such as Pterotrachea, where 

 each statocyst contains a statolith supported on bunches of 

 cilia, the circumstances are altogether different, as experiment 

 has shown. Amongst Polychaet worms statocysts are known 

 in Sabellidae, some Terebellidae, Arenicola, Aricidae, and some 

 Alciopidae. In some cases cilia are found within the vesicles, 

 and statoliths are present (either secreted, or consisting of 

 sand grains from the exterior). In the Arenicola species, 

 however, the state of development of the ' statocysts ' varies 

 within very wide limits and it is difficult to express any opinion 

 about the function of these organs. They appear to develop 

 from invaginations of the ectoderm, but there is not the close 

 connexion with the development of the cerebral ganglia which 

 is so characteristic of Peripatus. Are these vesicles homologous? 



It is interesting to look at the condition of things amongst 

 the Tracheata. 



In none of the Tracheata do the organs of orientation take 

 the form of statocysts associated with the supra-oesophageal 

 ganglion. But in the development of the supra-oesophageal 

 ganglion of the Myriapoda it is certainly very striking that pit- 

 like depressions occur on the ventral surface which afterwards 

 become closed vesicles and later disappear (10). The same 

 thing is true of the Insecta and Arachnida (1 and 12). 



In the Crustacea there is, so far as I am aware, no evidence 

 of pit-like depressions of this kind during development. Stato- 

 cysts are found, but these are not at all homologous with the 

 organs we are considering and occur in very different situations. 

 Curiously enough, there is a striking exception. Thus, according 

 to Claus (4), two otocysts are found connected with the cerebral 

 ganglion in certain Amphipoda — the Platyscelidae. The 

 same author mentions two vesicles as of similar function in the 



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