106 



CAUDAL GILLS Of ZYGOPTElRlD LARVi«, 



Avhich the tough outer stratum occupies from 2-4//. The hypo- 

 dermis is formed of a i-egular layer of high pyramidal cells, 24// 

 or more in height, terminating upon an inner basement-mem- 

 brane. The nuclei of these cells are of normal size, and are 

 nearly all situated at the base of the cells, close to the cuticle. 

 These cells are moderately pigmented, chiedy along their bases 

 and around their nuclei. The boundaries between the cells are 

 not pigmented. 



Internally, the hypoderm-layer completely bounds the reduced 

 cavity of the gill, which is chieHy filled with an alveolar mesh- 

 work of the usual kind. Supported in this meshwork, one can 



distinguish two blood-canals, 

 small in many sections, but in 

 some places swollen out and con- 

 taining much blood. Longi- 

 tudinal nerves are either absent, 

 or so fine as not to be traceable 

 in sections. No tracheae of large 

 size are visible, but, in each sec- 

 tion, five or six tracheae of smaller 

 calibre are cut; these would ap- 

 pear to represent the reduced 

 main longitudinal tracheal sys- 

 tem with its branches. 



Taking into consideration the 

 great increase in thickness in the 

 cuticle and hypodermis, and the 

 marked reduction of the tracheal 

 system, it would seem quite 

 justifiable to class these gills as a separate type, under the name 

 proposed above. It would l^e interesting to section the anal 

 pyramid of a large ^Eschnid larva, so as to compare the internal 

 structure of that organ with gills of the type here discussed. 



Text-tig. 32.* 



* Actual T.S. across interior of median gill of Agriou astdia- Perkins, 

 tor comparison with Text-fig. 81. Hypodermis and cuticle omitted. 

 ( X 258). Lettering as on p. 109. 



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