24 



radial muscles occurs in Tomocenis, Orchesdla and other 

 Collembola ; tliey are termed tlie dihdores liharyiigil, aud 

 act in coiijuuctiou with the circular iibres aud convert the 

 pharynx into a kind of pumping organ. 



The mid-gut commences in the posterior end of the 

 prothorax, and extends backwards into the fourth 

 abdominal segment, where it is joined by the hind-gut 

 (Plate lY., fig. -35). The lumen of the mid-gut is 

 practically circular in section, but its diameter narrows 

 considerably in the posterior fourth of its course. In 

 regard to its structure, it is very uniform throughout its 

 length, and its Avails consist of a layer of large epithelial 

 cells resting externally on a relatively stout tunica 

 jjropria, and on the outside of the latter is a system of 

 circular and longitudinal muscle fibres, both of which are 

 markedly striated (Plate Y., fig. 49, and Plate YI., fig. 61). 

 The epithelial layer consists of a row of very regular 

 cubical cells, which have a tendency to become somewhat 

 columnar. The boundaries between adjacent cells are 

 best seen towards the posterior region of the gut ; over 

 the rest of the area they vary greatly in their degree of 

 definition, and for the most part are hard to distinguish, 

 but their distinctness depends a good deal upon the 

 particular phase of physiological activity the cells hapjDeu 

 to be in at the time of killing and fixation. Each cell 

 contains a large oval or rounded nucleus situated near to 

 its centre ; the chromatin granules of the nucleus are 

 congregated together into several large masses (figs. 30 

 and 49). Towards the base of each cell there is, as a 

 rule, a deeply-staining granule of chromatic material 

 (ch. b in fig. -iO) ; the significance of this body is 

 unknown. The free margins of the mid-gut cells have a 

 finely striated appearance, which forms a " striated hem "' 

 or '' Harehensaum " (_hs. in fig. oU). 



