64 DK J. H. ASHWORTH 



which is directed anteriorly, lies near the lateralis muscle. Several minute statoliths 

 were present in life, forming a small mass in the centre of the statocyst. The wall 

 of the statocyst consists of two layers : an epithelium with large nuclei similar to the 

 epithelium of the neighbouring region of the mantle, and a thin enveloping layer of 

 ccelomic epithelium (fig. 18). The statocyst is in contact with the body-wall at the 

 angle of union of the latter with the dorsal mantle, and in this angle is a distinct 

 depression in the external epithelium suggesting that the statocyst had been formed 

 by invagination at that point. Professor Blochmann holds that the organs inter- 

 preted as statocysts are really the funnels of nephridia,* and describes and figures 

 an " Ausftihrungsgang." There can, however, be no doubt that the organs in 

 question are statocysts, .and they are closed sacs. Dr Yatsu (1905, p. 563) states 

 that the statocysts " must become smaller at the time of attachment " of the larva, 

 but his fig. 2 from a young attached Discinisca l&vis shows a statocyst considerably 

 larger than that of a free-swimming Pelagodiscus atlanticus."\ 



The mouth is situated immediately behind the epistome by which it is overhung ; 

 its cavity is lined by ciliated cells. The oesophagus, which has a high columnar 

 ciliated epithelium, passes at first dorsally and, after a short course, turns through 

 a right angle to run posteriorly. It opens, at the level of the body-wall, into the 

 mid-gut by a somewhat constricted aperture. The mid-gut is widest posteriorly ; 

 it represents the stomach and the digestive gland or "liver," the lobes of the latter 

 not having yet been formed. The cells of the wall of the mid-gut are, however, 

 sharply differentiated into two kinds : high epithelial flagellated cells which will 

 form the wall of the stomach, and vacuolated cells which will be included in the 

 future lobes of the "liver" (fig. 17). The high epithelium characteristic of the 

 stomach is present in (1) the anterior wall of the mid-gut, but only for a short 

 distance around the point of entrance of the oesophagus ; (2) in the middle and 

 posterior portions of the ventral wall ; and (3) in the ventral half of the posterior 

 wall of the mid-gut. The. remainder of the mid-gut, comprising the entire dorsal 

 and lateral walls, the dorsal half of the posterior wall, and the anterior portion 

 of the ventral wall, will form the " liver "-lobes. The epithelium of the stomach, 

 especially that of the posterior ventral region, is remarkable for the height and 

 slenderness of its cells, which are 11-15//, long, but not more than lfx wide at their 

 distal ends. The elongate, deeply staining nuclei are situated in the proximal 

 portions of the cells. Each cell bears only a single flagellum about 8-1 2/x long, which 

 arises from a well-marked basal granule in the distal region of the cell (fig. 19). 

 Many of the " liver "-cells are highly vacuolated; the remaining cells are filled with 

 spherules or less regular masses of secretion. The nuclei are at the bases of the cells, 

 and cell-outlines cannot be distinguished. The " liver "-tissue is similar in structure to 



* See also footnote, p. 48. 



t In Liwjula the statocysts do not diminish in size when the animal hecomes sedentary, as may be seen from the 

 figure of a statocyst of a sedentary example with shell 65 mm. long given by Dr Yatsu (1902«, fig. 22). This 

 statocyst is about three times as large as those of the larvae described on p. 51. 



