HIRUDINEA. 629 



The alimentary canal consists of a pharynx, oesophagus, proven- 

 triculus, digestive stomach, intestine, and rectum. The pharynx has mus- 

 cular walls, with the muscles arranged as radiating and circular fibres, very 

 largely developed in the Rhynchobdellidae, where it and the part of the body 

 which lodges it, constitute the retractile proboscis characteristic of the 

 group. In the typical Gnathobdellidae the radiating muscles of the pharynx 

 form three prominent ridges which have their cuticle produced into the 

 three well-known toothed jaws. The Australian Land Leeches (Geobdella, 

 Whitman) possess only the two lateral jaws. In other cases, e. g. Trocheta, 

 the jaws are quite rudimentary, or even, as in Leptostoma edentulum, lost 

 altogether 1 . There is an oesophageal tube, short in Hirudo, longer in most 

 other Leeches, thrown into folds in Clepsine when the proboscis is retracted. 

 The proventriculus is a feebly muscular tube with lateral paired caeca in 

 most Hirudinea, the number of pairs not being the same in all instances 2 . 

 The last pair bends backwards parallel to the axis of the body. Aulostoma 

 and one or two others possess this last pair alone. Pontobdella has a single 

 median ventral caecum underlying the stomach, whilst Trocheta and 

 Nephelis are devoid of caeca altogether. The stomach is a relatively short 

 tube : its calibre varies and is sometimes very small. In Clepsine it 

 has four pairs of lateral caeca. It is followed by a short intestine and a 

 rectum which ends in an anus placed dorsally and in front of the sucker, 

 except in Acanthobdella, where it lies in the sucker itself. 



The circulatory system of the Rhynchobdellidae consists of four longi- 

 tudinal vessels with muscular walls, viz. a dorsal, double in Branchellion, a 

 ventral and two lateral, one on either side. In Branchellion a branch from 

 each lateral vessel passes outwards into the first pair of branchial leaflets, 

 and into every third succeeding pair. It ends by an open mouth in a dila- 

 tation which communicates with the lateral coelomic sinus. Similar but 

 rudimentary dilatations are present in Pontobdella, Piscicola, and Clepsine. 

 In the Gnathobdellidae the pair of lateral vessels are alone present. The 

 vascular system is connected with the coelome in the Rhynchobdellidae pro- 

 bably only in the branchial dilatations or their rudiments ; in the Gnathob- 

 dellidae with the spaces of the botryoidal tissue (p. 218), and through them 

 with the remnants of the coelome. The coelome is much restricted by a 



of elongated hypodermis cells. In the eyes the vacuolated cells become very numerous ; they are 

 surrounded externally by a coat of pigment cells, and the axis of the eye is occupied by what appears 

 to be a string of hypodermis cells detached from the cap of short hypodermis cells which covers the 

 centre of the organ superficially. In Clepsine, according to the same authority, there are six dorsal 

 and four ventral rows of segmental papillae. The term goblet-organ is a misnomer. The centre of 

 all three sets of organs projects naturally, but it may be and is depressed by their retraction. The 

 eyes and segmental organs are, so far as can be judged, visual organs in the sense of appreciating the 

 difference between light and shade, and they are perhaps also tactile. The goblet-organs of the 

 lips are probably both gustatory and tactile? See p. 215, ante. 



1 See Whitman, op. cit. p. 388. 



2 The caeca extend into the branchial folds of Lophobdella. 



