IXODOIDEA 



697 



and contraction, and brought about by the contraction and relaxation of 

 muscles taking origin in the sclerites forming the endoskeleton of the head, 

 and inserted into these plates. It ends posteriorly in a narrow oesophagus, 

 which is without chitinous support, and therefore soft. This oesophagus 

 runs backward, perforating a large ganglionic mass, and ends in the mid-gut. 

 Its walls are composed of a layer of columnar cells internally and by muscular 

 fibres externally. Just before it joins the mid-gut its wall thickens and forms a 

 fold, the homologue of the proventriculus, which projects into that passage. 



The oesophagus opens on the floor of the large central food reservoir or 

 mid-gut, the main canal of which runs forward a little distance in front of 

 this opening, and backwards to the neighbourhood of the rectum. 



The central canal gives off diverticula, which may be classed into an anterior 

 set, consisting of a single diverticulum; a lateral set, consisting of an antero- 



FiG, 314. — Copulation of the Male and Female Tick. 

 (After Sambon.) 



lateral diverticulum, subdivided into three branches — a medio-lateral, irrlo 

 two or three, a postero-lateral, which is single, and a posterior set, whicli is 

 also single. 



The walls of the central tube and the diverticula consist of a single layer of 

 large cells lying upon a thin basement membrane, external to which are large 

 single muscular fibres arranged longitudinally and transversely in an open 

 network. Digestion is assisted by free cells mingling with the coagulated 

 blood filling the diverticula. 



Posteriorly the central canal is connected with the rectum by means of a 

 very fine canal, which appears to represent a functionless rudimentary intestine. 

 This communication is not admitted by Bonnet. 



Into the rectum open also two long, fine, much-convoluted tubules — the 

 Malpighian tubes, which are composed of a single layer of large cells placed 

 on a basement membrane. In females they may contain spherical corpuscles, 

 which, though present in males, are much smaller. Berlese and Bonnet 

 consider that these bodies are composed of guanine. 



The rectum is a sacculated tube, with a wall composed of a single layer of 

 flattened cells leading to the anus. Bonnet objects to the term ' anus,' 



