Statocysts of Arenicola 67 



Prof. Eetzius found nerve-endings), and, possibly, the pits present on 

 certain of the chaetiferous annuli of A. loveni and cristata (p. 35). 



The statocysts are the most highly developed sense-organs of 

 Arenicola. They are present in all the known species, except 

 A. pusilla, and afford valuable help in specific work. 



The statocysts of the lugworm were the first statocysts observed 

 in Annelids : they were discovered, in 1838, by Grube, who, how- 

 ever, mistook them for ganglia, but Stannius (1840) and von Siebold 

 (1841) at once recognised the analogy between these organs and the 

 statocysts of Molluscs. Meissner (1857), in a note x long overlooked, 

 first showed that the statocysts of the lugworm open to the exterior 

 by means of a canal. During the last twenty years the statocysts of 

 Arenicola have been the subject of researches by Profs. Ehlers and 

 Fauvel and Drs. Gamble and Ash worth. 



The statocysts are situated a short distance from the brain, external 

 to the dorsal or dorso-lateral portions of the oesophageal connectives 

 (PL XIII, Fig. 46). Each statocyst is primarily an invagination of 

 the epidermis, and in A. marina, assimilis and glacialis the connection 

 with the epidermis and the exterior is maintained by means of a 

 narrow bent tube, the minute aperture of which may be found near 

 the origin of the metastomial groove (see p. 39). The walls of the 

 statocyst and tube are composed of sensory and epithelial cells ; 

 some of the latter are glandular and secrete the thin cuticle which 

 lines the vesicle and the tube. Each sfcatocyst contains one or more 

 statoliths, and a fluid composed of a mixture of sea water and the 

 secretions of the gland cells in the wall of the organ. 



It is important to note that, in those species in which there is a 

 tube leading from the statocyst to the exterior, the shape and nature 

 of the statoliths vary considerably in different specimens of the same 

 species, according as the tube is functionally open or closed. This 

 has been shown, by the present writer, to be the case in both 

 A. marina and A. assimilis var. affinis. Numerous specimens of the 

 former species, from different localities, have been examined. In 

 those in which the statoliths consist of many foreign bodies, such 

 as quartz-grains, portions of sponge-spicules, frustules of diatoms, etc., 

 practically without any secreted covering (PL XIV, Fig. 47), the tube of 

 the statocyst was found to be open ; in others in which the original 

 statoliths had become covered with layer upon layer of pale-yellow 

 secreted substance (Fig. 37 A) the tube was found invariably to have 



1 Zeits. ration. Med., 3 Beihe, i. (1857), p. 635 f.n. 



F 2 



