PERLIDAE 



403 



perennibranchiate Perlidae are as conspicuous as they are in 



the exceptional Pteronarcys : for it appears that at the final 



moult the gills usually become very 



much contracted and concealed by the 



new integument ; in some cases they 



merely appear as slight prominences 



in the neighbourhood of the stigmata. 



Pictet, Dufour, Newport, and Imhof ^ 

 have studied the internal anatomy. The 

 aUmentary canal is remarkable for the 

 enormous oesophagus; there is no dis- 

 tinction between this and the crop. A 

 proventriculus is quite absent, and there 

 are no chitinous folds in the position it 

 usually occupies. The true stomach is 

 small, and only commences in the fourth 

 abdominal segment. It has a prolonged 

 lobe on each side in front, and in 

 addition to this eight sacs ; thus there 

 are formed ten diverticula, fastened to 

 the posterior part of the oesophagus by 

 ligaments. The terminal portion of the 

 stomach is small, and apparently only 

 distinguished from the short intestine 

 by the point of insertion of the Mai- 

 pighian tubes; these vary in number ^^'i^-^ltt^ TV^L 



maxima. (After Imhof.) /, 

 Upper lip ; mli^ buccal cavity ; 



from about twenty to sixty. There are 

 two pairs of large salivary glands. In 

 Pteronarcys the caecal diverticula of the 

 stomach are wanting. In some Perlidae 

 the terminal parts of the gut are more 

 complex than in Perla onctxima ; New- 

 port figures both an ilium and colon 

 very strongly differentiated, and states 

 that these parts differ much in Perla and Pteronarcys. Accord- 

 ing to him the stomach is embraced by a network of tracheae, 

 and Imhof tells us that he found the stomach to contain only air. 

 The brain is small, but, according to Imhof, consists of four 

 amalgamated divisions; the infra-oesophageal ganglion is small, 



' Beitr. Anat. Perla viaxima. Inaug. -Diss. Aarau, 1881. 



ap, common termination of 

 salivary ducts ; 0, oeso- 

 phagus : s, salivary glands ; 

 ag^ duct of salivary gland ; 

 It, anterior diverticula of 

 stomach ; Ig, their ligaments 

 of attachment ; inp, Malpi- 

 ghian tubes ; r, rectum ; af, 

 anal oriiice. 



