40 NOTES ON THE ANATOMY OF DTNOPHILCS. 



ring becomes much broader in approaching the ventral surface, and 

 that it becomes indistinguishable from an investment of cilia which 

 clothes the ventral surface of the head and which passes continuously 

 into the ciliated lining of the oesophagus (cf. fig. 3). The examina- 

 tion of the ciliation of the ventral surface of the head is always 

 difficult in fresh specimens, but at the time when these were acces- 

 sible to me, I believed that I could convince myself that the anterior 

 circlet of the second prseoral ring passed completely round the head, 

 as shown in fig. 15. The most satisfactory way, it appears to me, 

 of reconciling the apparent discrepancy between fig. 3 and fig. 15, 

 is to assume that, whilst the anterior circlet of the second prseoral ring 

 does really pass continuously round the ventral surface of the head, 

 the middleand posterior circlets become, ventrally, an extensive ciliated 

 area which is continuous with the ciliated lining of the oesophagus. 



The arrangement of the five pairs of ciliated rings which occur on 

 the body and of the perianal ring is sufficiently explained by fig. 1. 

 All these rings are interrupted by the cilia which cover, in a uniform 

 sheet, the entire ventral surface of the body and of the tail. 



Alimentary Canal — The mouth occurs on the ventral surface, at the 

 limit between the head and the first segment of the body. The 

 aperture of the oesophagus is guarded by two lip-like structures, an 

 outer and an inner. Of these, the former constitutes the outer wall 

 of a triangular space (fig. 15) which includes in front the aperture 

 into the oesophagus, and behind the end of the tongue-like structure 

 formed by the muscular appendage of the oesophagus. The arrange- 

 ment of this organ is well seen in the longitudinal section figured (fig. 

 3), where it will be noticed that the end of the muscular appendao-e 

 (which is covered by a modified, probably hardened epidermis) pro- 

 jects into the space enclosed by the outer lip. A similar arrangement 

 is figured by Eepiachoff (No. 12, pi. iv, fig. 1) in D. gyrociliatus, whilst 

 the disposition of the organ appears, from Weldon's description (13), 

 to be somewhat different in D. gigas. 



In front of the tongue-like structure is seen the aperture into the 

 oesophagus (fig. 1 5) . This aperture is subtriangular, and is bounded 

 by the two richly ciliated inner lips. 



The course of the alimentary canal is shown in fig. 3. The 

 oesophagus ascends obliquely towards the dorsal surface, the lateral 

 walls of its first part being thickened {v. fig. 10), and passing con- 

 tinuously into the inner lips. The posterior section of the oesophagus 

 lies very near the dorsal skin, and is lined by cells which have a 

 more glandular appearance, and which bear longer cilia than those 

 which line the anterior two thirds of the oesophagus. The posterior 

 division corresponds to the proventriculus (" Vormagen ") described 

 by Korschelt in D. ajpatris. 



