1901. J NATURAL SCIENCES OF rillLADELPIIIA. 129 



Development of the Gizzard. 



The gizzard is a specially thickeued aud cuticularized portion of 

 the alimentary canal, appearing at the junction of fore- and mid- 

 gut, and projecting slightly into the latter. Externally is a coat 

 of circular muscle fibres, thicker in the larvaj than in the adult 

 and varying in thickness in adults of different genera. Within is 

 a layer of epithelial cells, at first one cell in thickness, but later 

 becoming many cells thick in definite areas, to form folds project- 

 ing into the lumen of the canal. 



In the forms examined, four folds or some mvdtiple of four to 

 as many as thirty-two have been found — sixteen and eight being 

 the numbers most commonly occurring. 



From the epithelial cells is developed the inner chitinous coat of 

 the gizzard, which is thin on the spaces between folds, becoming 

 thickened on the folds aud forming here horny teeth of various sizes. 



The writer has examined Agrionine larvie from the time of 

 hatching to the period immediately preceding metamorphosis. 



In larvffi just hatched no evidence of any gizzard-armature was 

 found. 



The following statements refer to larvte of Ischnura verticalis 

 Say: 



In larviB 3 and 4 mm. in length' the gizzard is quite clearly 

 marked (fig. 25). The chitinous coat shows sixteen folds or fields, 

 eight larger alternating with eight smaller. Each larger field is 

 armed with teeth arranged in two groups, an anterior and a pos- 

 terior, the latter occupying about the centre of the gizzard. The 

 posterior group consists of two large pointed teeth, each enclosed 

 on its outer side by four to five small teeth. The anterior group 

 comprises two pointed narrow teeth intermediate in size between 

 the two sets mentioned above. 



Each smaller field has one group of three to four sjuall teeth, 

 ■which are at the same level as that of the posterior group of the 

 larger fields. 



A larva 6 mm. long shows practically the same armature, but 

 on the smaller fields also there are often two groups, the anterior 

 comprising one to two teeth, the posterior three to four. 



' In all of these larvae, by length 1 meau the distance measured from the 

 external anterior part of the head to the posterior limit of the last abdom- 

 inal segment, the gills being excluded. 



