62 PROCEEDINGS OF THE MALACOLOGICAL SOCIETT. 



The gill (PI. VI, Fig. 2, g.) is very large, extending throughout 

 the whole of the mantle-cavity ; the gill-filaments of which it is 

 composed are exceptionally long, and are partially attached to the 

 mantle, the unattached portion projecting freely into the mantle- cavity. 

 The osphradium (o.s.) is a long rod-shaped body lying underneath the 

 gill, and nearly coextensive with it; both gill and osphradium are 

 very similar to those of Cerithium. 



The floor of the mantle-cavity is beset with ridges which consist of 

 glandular mucous- secreting tissue. 



The nervous system.— Yon. Jhering [10] has figured and described 

 the nervous system of T. communis, Eisso, whilst Bouvier |_1] has 

 recently investigated that of T. rosea, Quoy & Gaim. I find that, 

 though the description given by the former is accurate, yet his 

 figure [10] (pi. x, fig. 5) is somewhat misleading, especially in 

 respect to the relative size of the ganglia and length of the 

 commissures. 



The cerebral ganglia {e.g.) (PL VI, Figs. 3 and 4) are pyriform in 

 shape, and are united to one another underneath by a short, broad 

 commissure. The pleural ganglia {pig., pl'.g-) lie slightly under, 

 and are attached to the cerebrals by very short connectives. Two 

 oval bodies, closely connect d to one another internally, are situated 

 anteriorly and ventrally to the cerebrals ; these are the pedal 

 ganglia [pd.ff.). 



The cerebral and pleural ganglia are joined to the pedals by 

 connectives, the pleuro-pedal being about twice as thick as the 

 cerebro- pedal. 



A visceral nerve is given off from the left pleural ganglion ; it 

 runs underneath the oesophagus towards the right side, passing into 

 the sub-intestinal ganglion (si.g.), which lies to one side of and 

 slightly underneath the right pleural, and is joined to it by an 

 extremely short connective (z.), the inter - visceral commissure of 

 Von Jhering [10]; we have thus a condition of zygoneury on the 

 right side. 



The sub-intestinal ganglion, after giving off a pallial nerve (which 

 almost immediately bifurcates, one branch innervating the upper 

 part, the other the lower part of the mantle), is continued into 

 the sub-intestinal branch of the visceral commissure, which, running 

 parallel with the oesophagus to the end of the mantle - cavity, 

 terminates in the abdominal ganglion (ab.g.). The right pleural 

 ganglion gives off a visceral nerve that passes obliquely over the 

 oesophagus and is continued into the supra-intestinal ganglion {sp.g.) ; 

 from the latter the supra intestinal branch of the visceral commissure 

 is given off ; this runs beside the osphradium to the end of the 

 mantle-cavity, and terminates in the abdominal ganglion. 



From the left pleural ganglion a pallial nerve is given off to the 

 mantle. The supra intestinal ganglion gives origin to two branchial 

 nerves, the anterior of which runs parallel to the left pallial nerve, 

 and at the base of the mantle anastomoses with the latter by means 

 of a very fine transverse commissure ; hence on this side we have 

 a condition of dialyneury (<(.). 



