390 - THE CRAYFISH CHAP. 



divides into (a) a ventral thoracic artery supplying the segments and 

 appendages of the thorax, and (b) a ventral abdominal artery, sup- 

 plying 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 diges- 

 tive gland on either side of, and extending further back than the 

 gizzard ; above it are a 7 , the gonads, on either side of and behind the 

 pericardial 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 month, bounded by the 

 labruin 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. 



