106 THE FRESH-WATER CRAYFISH-THE CAPE CRAWFISH 



the internal structure. The gastric mill contains ossicles and teeth, and in the 

 pyloric portion there is a sieve of fine hairs. 



C. Tlie Reproductive System. 

 In the Female note : — 



■ (a) The ovary (when ripe) is a Y-shaped organ, consisting of two anterior 

 lobes and a posterior lobe. [In Jasus the ovaries are a pair of long 

 reddish organs, lying on either side of the alimentary canal, and united 

 by a median transverse bridge.] 

 (b) The oviducts, a pair of short straight tubes, one leading from each lobe 

 to open at the genital aperture on the third walking leg. 



In the Male note : — 



(a) The testis, which is three-lobed. like the ovary, and similarly situated. 



[In Jasus the testes are a pair of elongate white organs, connected together 

 like the ovaries, and in a similar position.] 



(b) The vasa deferentia, a pair of thick, coiled, white tubes, each given off 



at the junction of the three lobes, and opening to the exterior at the 

 genital aperture on the last walking leg. 

 Make a complete drawing showing the vascular, alimentary, reproductive 

 and muscular systems. Remove, and draw, the gonad (male or female) with 

 its ducts. 



D. The Skeletal System. 



Remove all the muscles from the cephalo-thorax and the abdomen so as to 

 expose the internal skeleton, taking care not to break the sternal artery. The 

 ventral and lateral regions of the thoracic exoskeleton are produced into the 

 interior of the body in the form of a segmental series of calcified plates, so 

 arranged as to form a row of lateral chambers in which lie the muscles of the 

 limbs, and a median tunnel-like passage or sternal canal, containiag the thoracic 

 portion of the nervous system. This entire endophragmal system, as the series 

 of plates is called, constitutes a kind of internal skeleton. 



E. The Nervous System. 



Expose the nerve cord in the abdominal region and trace it forwards. In 

 this region note the six abdominal ganglia on the nerve cord. The sixth 

 ganglion is the largest and represents the ganglion of the 6th abdominal seg- 

 ment and that of the telson fused together. The pairs of gangUa are fused, and 

 the double nerve cord is fused in the abdominal region. In the cephalothorax 

 the paired nerve cord lies in the sternal canal of the endophragmal skeleton. 

 In order to expose it, cut through the sternal canal on each side and remove the 

 roof. Note the sub-oesophageal ganglion, a fused mass consisting of the three 

 posterior cephaUc and the three anterior thoracic ganglia. Leading from the sub- 

 oesophageal ganglion, on each side of the oesophagus, is the circum-oesophageal 

 commissure connecting it with the supra-oesophageal ganglion situated at the 

 base of the antennules. This ganglion sends nerves to the eyes, antennules and 

 antennae. The sub-oesophageal ganglion siipplies the mandibles, maxillae, and 

 the first and second maxillipeds. In each remaining segment of the cephalo- 

 thorax is an indistinctly-double ganglion sending nerves to the third maxillipeds 



