390 THE CRAYFISH CHAP. 



compare Figs. 93 and 94) : it perforates the ventral nerve- 

 cord, and divides into (a) a ventral thoracic artery supplying 

 the segments and appendages of the thorax, and (b) a ventral 

 abdominal artery, supplying the segments and appendages of 

 the abdomen (this artery can be seen in injected specimens 

 through the transparent cuticle). 



5. Note the position of the following parts before dissecting 

 further : a, the gizzard, a large sac in the head, with two 

 pairs of muscles passing to the integument (now cut through) ; 

 (b), the adductor muscles of the mandible, just external to a ; 

 c, the paired, brownish or greenish digestive gland on either 

 side of, and extending further back than the gizzard ; above 

 it are d, the gonads, on either side of and behind the peri- 

 cardial sinus. In the male, the spermary is small and whitish, 

 and each spermiduct is a coiled, densely white tube ; in the 

 female, the ovary is a larger, brownish organ, containing 

 prominent ova. (In both sexes, the paired character of 

 the gonads is partly lost by fusion : a pair of anterior lobes 

 and a single posterior lobe can be seen in each.) Sketch. 

 By slightly raising the surrounding parts the gonaducts 

 can be seen to pass ventralwards to their external apertures 

 (p. 388), the oviducts being thin-walled and straight. White 

 masses, the spermatophores (p. 382), will very likely be found 

 stuck on to the sternal region of the body. 



6. Tease up a small portion of the spermary or of a 

 spermatophore ; stain, and mount in glycerine. Examine 

 under the microscope and note the rounded and flattened 

 sperms, each with a number of stiff, curved processes coming 

 off from the periphery. The sperms are non-motile. Sketch. 



Remove the heart and reproductive organs carefully, 

 noting the sternal artery (see above) as you do so, and taking 

 especial care not to injure the surrounding parts. Examine 

 the heart under water, and note the six ostia. 



II. The enteric canal. Note i. The oval mouth, bounded 

 by the labrum in front, leading into a short and wide gullet 

 (this will be seen later on), which dilates to form the large 

 gizzard (Fig. 93), filling up a considerable portion of the head 

 and extending into the thorax : a transverse constriction 

 divides it into an anterior and a posterior portion ; both 

 gullet and gizzard are lined by chitin. The chitinous 

 cuticle of the gizzard is calcined in places to form the sclerites 

 or so-called " ossicles " of the " gastric mill." Note the 

 two median sclerites bounding the transverse constriction 

 in front and behind respectively : to them the anterior and 

 posterior pairs of muscles (mentioned in 5) are attached. 



