ARTHROPODA 295 



thin walls, and acts as the branchia. The last pair of append- 

 ages project backward like caudal processes. 



The mouth is situated far forward, and the short oesophagus 

 lined with chitin has a nearly horizontal course. The stomach 

 is small, and lies almost entirely in the cephalothorax ; it is 

 also lined with chitin, which is thickened at parts, and it con- 

 tains a number of hairs and teeth, which may act as strainers. 

 The intestine is a wide tube running quite straight to the 

 anus, which lies on the ventral surface of the posterior end of 

 the body. The liver consists of two pairs of lateral caeca, 

 which extend back as far as the intestine. They open by a 

 common duct on each side, at the junction of the stomach with 

 the intestine. 



There is a cerebral ganglion connected by commissures 

 with two fused pairs of sub-oesophageal ganglia, then come seven 

 pairs of ganglia corresponding with the seven free thoracic seg- 

 ments, and finally a complex ganglion formed of three or four 

 ganglia fused together, which supplies the abdomen. The com- 

 missures which connect the various ganglia are separate and 

 distinct. 



The elements which form the eyes are arranged on each 

 side of the head in four patches (Fig. 172); there do not 

 seem to be any auditory organs. Olfactory hairs exist on the 

 small anterior antennae. 



The heart is an elongated vessel situated in the dotsal 

 middle line ; it appears to be closed behind, but anteriorly it 



Fig. 171. — Asellus aquatiaus. Section througt 

 third thoracic segment. From Leuckart and 

 Nitsche, after G. 0. Sars. 



1. Anterior end of heart (aorta). 



2. Ovary. 



3. Alimentary canal. 



4. Hepatic diverticula. 



5. Nervous system. 



6. Base of thoracic limhs with inward projections 

 (oostegites) which form the floor of the brood- 

 poiich. , 



is prolonged into an aorta. Three pairs of ostia situated in 

 the last three thoracic segments open into the heart. The 

 widest part of the heart is situated in the abdomen ; the aorta 



