1922] Branch: Internal Anatomy of Trichoptera 267 



muscles have formed themselves into the characteristic six pairs and 

 those pushed to the outside of the outer circular muscles have ceased, 

 Plate XVI, Fig. 4. This heavy outer circular muscle comes to an end 

 in the seventh segment, leaving the six longitudinal pairs as the exterior 

 layer, Plate XVI, Fig. 3. This condition prevails throughout segment 

 eight, Plate XVI, Fig. 2, and past the semi- valve into the gill chamber 

 of the rectum, Plate XVI, Fig. 1. For a diagram of this musculature, 

 see Plate XVI, Fig. A, the levels drawn are numbered as the figures 

 which are taken in cross section at these same levels. The musculature 

 of L. indivisus does not vary from this description based upon H. 

 analis, with the exception that in the mesenteron the longitudinal 

 muscles are of a single layer, Plate XVI, Fig. S. The blood gills of 

 H. analis rightfully belong to the discussion of the histology of the 

 alimentary canal, but their structure is sufficient to claim for them a 

 separate division of this paper. 



Blood Gills of Hydropsychodes analis Banks. 



Protruding from the T-shaped anus, Plate XV, Figs. 8 and 9, 

 Plate XVII, Fig. 1, are often seen four transparent, greenish, finger-like 

 appendages, Plate XIV, Fig. 3. These are protruded beyond the 

 limits of the anus when the larva is out of the water crawling upon 

 the rocks, or may be artificially protruded by pressure upon the abdomen 

 in the caudal region. Pictet, 1834. states that these appendages contain 

 tracheae. There are muscles leading into the gills which might be 

 mistaken for tracheae if the microscope were not of high magnification. 

 Dufour, 1847, figures these gills as appendages from the rectum into the 

 body cavity, but without trachea?. This error of Dufour is easily under- 

 stood when the larva is dissected, for it is almost, if not quite, impossible 

 to open a larva from the dorsal side and not cut into the rectum so 

 that these gills float out as though appendages from it. It is only 

 when a lateral opening is made that the true internal position is observed. 

 Fritz Muller, 1888, does not show any tracheae in the gills and Thienman, 

 1903, and Lubben, 1907, represent these as non-tracheae bearing 

 structures. In the glycerine mounts of the caudal end of the abdomen 

 with the gills protruding from the anus, the tracheal tubes are seen to 

 pass down the sides of the body sending branches to the alimentary 

 canal and surrounding tissues. Long branches or continuations of the 

 main tracheal trunks extend into the anal pro-legs or drag hooks, 

 but in no instance does a single tracheole extend to a gill. In neither 

 the transverse sections or the longitudinal ones does tracheal structure 

 appear. Muscles and blood make up the content of the gill. These 

 gills are in direct communication with the body cavity and it is possible 

 that, when occasion renders the supply of oxygen insufficient, the blood 

 rushes from the blood sinuses into these " pockets" and comes with 

 sufficient force and quantity to extend the gill and push it to the exterior 

 and an exchange of gases takes place through the wall of the gill. This 

 action, combined with the lack of tracheae, seems sufficient evidence 

 upon which to claim the term of true blood ^ills for these structures. 



