V] THE ALIMENTARY AND EXCRETORY SYSTEMS 115 



by means of internal tracheal gills, developed in the anterior portion 

 of the rectum, which is greatly dilated to form a branchial basket 

 (fig. 44 B) of great complexity. As this organ belongs physiologically 

 to the respiratory system, we have dealt with it fully in chap. IX. 

 We shall only remark here that the gills are projections of the wall 

 of the rectum, involving the cuticle or intima, which covers them 

 completely, and the epithelial cells, which become specialized for 

 their respiratory function by fusion into a complete syncytium. 

 An enormous number of tracheal capillaries ramify in these gills. 

 The circular muscular tunic remains complete, except for penetration 

 of it by tracheae. The external longitudinal muscles are arranged in 

 six bands, as in all other portions of the hind-gut. Reduced remnants 

 of the gills persist in the imago (fig. 44, br), but the basal pads (p. 188 X 

 are cast off [109]. 



Fig. 51. T.S. through one of the three rectal pads, from larva of Aeschna brevistyla 

 Ramb. ( x210). Original. 



The hinder portion of the rectum, consisting of about one-third 

 or less of its entire length, remains undifferentiated, and exactly 

 resembles the same portion in the Zygopterid larvae. This part, 

 in the Anisopterid larvae, may be termed the vestibule, or stercoral 

 chamber (fig. 44 B). In the Aeschnidae the branchial basket narrows 

 posteriorly, and passes insensibly into the vestibule. In the Libellu- 

 lidae, a strong sphincter muscle definitely separates the two. A pos- 

 terior sphincter, in both cases, closes the vestibule just in front of 

 the anus. 



The epithelium of the vestibule carries anteriorly three longi- 

 tudinal pads, formed usually of high columnar cells (fig. 51). These 

 pads have for long been known as the rectal glands. As they are 

 obviously not of a glandular nature, we follow Sadones [145] in 

 rejecting this term, and shall adopt the term rectal pads. We 

 know nothing certain of their functions, any more than we do 

 concerning the pads in the small intestine. 



