Statocysts of Arenicola 69 



become closed, either by apposition of its walls or by blocking of its 

 lumen by granular substance secreted by the gland cells in the wall 

 of the tube. A striking instance of this latter condition was exhibited 

 by a specimen from Trieste, in which each statocyst (Fig. 37 B) con- 

 tained about forty statoliths, approximately spherical, nearly uniform 

 in diameter, and composed almost entirely of secreted chitinoid 

 substance, there being in each only a small central " nucleus." The 

 statocysts were completely shut off from the exterior, the tube of each 

 being closed by a plug of granular secretion. 



Similar conditions are met with in A. assimilis var. affinis. The 

 statoliths of examples from Otago Harbour, New Zealand, are 

 irregular bodies (Fig. 38 A) ; but those of specimens from near 

 Wellington, New Zealand, from Tasmania and from the Falkland 

 Islands (Fig. 38 B), are rounded, each being composed of a central 

 granule with a thick envelope of yellow secreted material. 1 The 

 canal of the statocyst is moderately widely open throughout its 

 length in the Otago examples, but in all the others is closed at one 

 or more points, generally by small masses of secretion of the same 

 refringent nature as that composing the statoliths. In each statocyst 

 of the two Falkland specimens examined there are two statoliths larger 

 than the rest. They were the first statoliths of the post-larval stage. 



These observations on A. marina and A. assimilis var. affinis 

 indicate clearly that the nature of the statoliths depends on whether 

 actual open communication with the exterior is or is not maintained. 

 The form of the statoliths in those species in which a canal leads 

 from the statocyst is variable, and is therefore a character of little 

 value in systematic work. 



Only one statocyst of A. ylacialis is available for examination. 

 The lumen of the tube which connects the statocyst to the exterior 

 is open throughout its length, and the 

 statoliths are numerous naked sand- 

 grains (Fig. 39). 



In A. cristata, loveni, branchialis 

 and ecaudata the connection of the 

 statocyst with the epidermis and ex- Fig . ^^ 

 terior has been lost, and the organ has 3 ^ ST ' T ' 



become a closed, oval or spherical sac, 

 in which the statolith or statoliths are spherical, oval or lenticular 

 chitinoid bodies. In A. cristata (Fig. 40) and loveni there is a 



1 The statoliths are of this latter type also in two specimens of A. assimiUs 

 from Uschnaia. 



