ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



305 



and hypodermis, with tracbeje, two blood-canals, nerves and alveolar 

 tissue, formed from hypoderm cells growing into the htemocoele. Each 

 gill is attached by a basal piece, with a breaking-joint. Tillyard dis- 

 tinguishes (1) the saccoid type, more or less sausage-shaped, either 



mr 



Ideal (Semi-diageammatic) T.S. of Median Caudal Gills to show 

 THE Internal Structure of the Gills. 



A. Across the saccoid gill-system of Diphlebia. 



B. Across the vertical lamellar gill-system of Austroagrion {Agrionid 

 form of gill). 



al, alveolus : cu, soft inner layer of cuticle ; dh, dorsal blood-canal ; 

 dl, principal dorsal internal lamina ; dn, dorsal longitudinal nerve of 

 gill ; hy, hypodermis ; il, internal lamina ; tr, main longitudinal trachea 

 of gill ; tr', branch trachea of gill ; vh, ventral blood-canal ; vl, principal 

 ventral internal lamina ; vn, ventral longitudinal nerve of gill. 



simple or constricted ; (2) the triquetro-quadrate type, triangular m 

 transverse section ; (3) the lamellar type, with a rachis and vertical or 

 horizontal blades, but admitting of further classification ; and (4) the- 

 reduced (non-functional type), with a vanishing tracheal system. 



J. A. T. 



/3. Myriopoda. 



Peculiar Variation in Brachydesmus. — Henky W. Beolemann: 

 (Ann. Mag. Nat. Hist., 1918, 1, series t), 281-J:, 3 figs.^ describes a 

 peculiar condition in an immature specimen of a species oiBrachydesmus — 

 namely, the presence of almost adult copulatory organs. Such a structure- 

 is frequent and even normal with Diplopods of archaic type, such as- 

 Colobognatha or Spiroboloidea ; but that it should be witnessed in the 

 highly specialized Polydesmoidea is certainly most striking. It seems- 

 to be an instance of " neotania." J. A. T. 



