152 



HARDWICKE'S SCIENCE-GOSSIP. 



libellulid larvae, which, as is well known, respire by 

 admitting fresh supplies of water into this cavity. It 

 is an obvious objection that cockroaches and other 

 insects in which the rectal bands are well developed 

 do not take water into the intestine at all. Gegen- 

 baur has therefore modified Leydig's hypothesis. 

 He*suggests, (Grundzuge d. Vergl. Anat.) that the 

 functional rectal folds of dragon-flies and the non- 

 functional folds of terrestrial insects are both survivals 

 of tracheal gills, which were the only primitive 

 organs of respiration of insects. The late appearance 

 of the rectal folds and the much earlier appearance 



related to the six theoretical elements (two tergal, 

 two pleural, two sternal), traceable in the arthropod 

 exoskeleton, of which the proctodeum and stomo- 

 daeum are reflected folds. 



The anus of the cockroach opens beneath the tenth 

 tergum, and between two " podical " plates. Anal 

 glands, such as occur in some beetles, have not been 

 discovered in cockroaches. 



The three principal appendages of the alimentary 

 canal of the cockroach are outgrowths of the three 

 primary divisions of the digestive tube ; the salivary 

 glands are diverticula of the stomodseum, the ca^cal 



Fig. 79. — Transverse section of gizzard oi cock- 

 roach. The chiiinous folds are represented here 

 as symmetrica!. See next figure. X 40. 



Fig. 78.— Alimentary canal of cockroach. X 2. 



Fig. 80. — The six primary folds (teeth) of the gizzard, seen in profile. 



of spiracles is a serious difficulty in the way of this 

 view, as Chun has pointed out. It seems more 

 probable that the respiratory appendages of the 

 rectum of 'the dragon-fly larvae are special adaptations 

 lo aquatic conditions of a structure which originated 

 in terrestrial insects, and had primarily nothing to 

 do witli respiration. 



The number of the rectal bands (six) is worthy of 

 remark. We find six sets of folds in the gizzard and 

 small intestine of the cockroach, and six longitudinal 

 bands in the intestine of the lobster and crayfish. 

 The tendency to produce a six-banded stomodaeum 

 and proctodasum (if not fanciful) may possibly be 



tubes of the mesenteron, and the malpighian tubules 

 of the proctodaeum. 



A large salivary gland and reservoir lie on each 

 side of the oesophagus and crop. The gland is a thin, 

 foliaceous mass about \ in. long, and composed of 

 numerous acini, which are grouped into two principal 

 lobes. The efferent ducts form a trunk, which 

 receives a branch from a small accessory lobe, and 

 then unites with its fellow. The common glandular 

 duct thus formed opens into the much larger common 

 receptacular duct, formed by the union of paired 

 outlets from the salivary reservoirs. The common 

 salivary duct opens beneath the lingua. Each 



