270 ZOOLOGY. 



The blood is driven bj the heart through the axteries, and 

 a large part of it, forced into the capillaries, is collected by 

 the Tentral venous sinus, and thence passing through the 

 gills, where it is oxygenated, returns to the heart. 



The giUs are appendages of the three pairs of maxilli- 

 pedes and the five pairs of feet, and are contained in a 

 ohamber formed by the carapace ; the sea-water passing into 

 the cavity between the body and the free edge of the cara- 

 pace is afterwards scooped out through a large opening or 

 passage on each side of the head, by a membranous append- 

 age of the leg, called the "gill-paddle"' {Scapliognathite). 



The digestive system consists of a mouth, opening between 

 the mandibles, an oesophagus, a large, membranous stomach, 

 with very large teeth for ci'ushing the food within the large 

 or cardiac portion, while the posterior or pyloric end forms 

 a strainer through which the food presses into the long, 

 straight intestine, which ends in the telson. The liver is 

 very large, dai'k green, with two ducts emptying on each side 

 into the junction of the stomach with the intestine. 



The nervous system consists of a braiii situated directly 

 under the base of the rostrum (supraoesophageal ganglion), 

 from which a pair of optic nerves go to the two eyes, and a 

 pair to each of the four antennee. The mouth-parts are 

 supplied with nerves from the infraoesophageal ganglion, 

 which, with the rest of the nervous system, lies in a lower 

 plane than the brain. There are behind these two ganglia 

 eleven others ; the cephalo-thoracic portion of the cord is 

 protected above by a framework of solid processes, which 

 forms, as it were, a ''false-bottom" to the cophalo-thorax ; 

 tliis has to be carefully removed before the nervous cord can 

 be laid bare. A sympathetic nerve passes around each side 

 of the a?sophagU; and distributes branches to the stomach. 



The nerves of special sense are the optic and auditory 

 ner^'es. The eyes are compound, namely, composed .of many 

 simple eyes, each consisting of a cornea and crystalline 

 cone, connected behind with a long, slender connective rod, 

 uniting the cone with a spindle-shaped body resting on or 

 against an expansion of a fibre of the optic nerve, and is 

 ensheathed by a retina or black pigment mass (Fig. 221 s^ 



