50 CAUDAL GILLS OF ZYGOPTKRID LARVAE, 



thrown off, the tracheae and blood-canals are torn away at their 

 narrowest portions, leaving an almost continuous sheet of hypo- 

 dermis to cover the wound. Before any blood could be lost, the 

 contraction of the muscles of the basal piece will have drawn 

 the wound together, so that the blood-canals and tracheie are 

 closed up. A rapid overgrowth of hypoderm-cells then takes 

 place, forming a bulge or tubercle upon which a new cuticle is 

 rapidly developed. From this bulge, by further growth of the 

 hypodermis, a regene7-ated giW will be reconstructed at the follow- 

 ing ecdysis. 



The Alveolar Meshivork (Plate iv., tigs.30-33). 



This peculiar structure tills the greater part of the interior of 

 the gill in the Saccoid and Triquetro-quadrate Types, and persists 

 also in the central area or rachis of the Lamellar Types. It was 

 first discovered and described by Dr. F. Ris, in his study of the 

 saccoid gills of Pseudophcea (28). A translation of his opening 

 sentences on this formation offers us an excellent word-picture 

 of it :— 



The interior of the bladder under the hypodermis is filled 

 with a very queer web-like body ("sehr eigentiimlichen Gewebe- 

 kbrper"): it is of quite regular alveolar construction, and the 

 individual alveoli are spheroids flattened against each other. 

 The alveoli possess genuine partition-walls ("achte Wandungen") 

 made from a substance which, as far as my optical aid (Leitz 

 Imm. 1/12) went, is structureless and only weakly colourable 

 with hsematoxylin; examination in series shows that in each 

 alveolus there lies one firm cell-nucleus, and only one, so that 

 the supposition arises that each alveolus is the product of a 

 single cell. 



A careful study of the alveoli in numerous sections through 

 gills of Diphlehia, Neosticta, and Calopteryx convinces me that 

 His' description is correct, except in one point. It is true, as a 

 general rule, that if one follows a single alveolus through a 

 number of sections, from the point at which it first appears as a 

 minute area between two larger alveoli, to the point at which it 

 finally disappears, one and only one nucleus can be seen upon its 



