ARTHROPODA 29 
ut 
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 les 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 gangha corresponding with the seven free thoracic seg- 
ments, and finally a complex ganglion formed of three or four 
gangha 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 dorsal 
middle line; it appears to be closed behind, but anteriorly it 
Fic. 171.—Asellus aquaticus. Section through 
third thoracic segment. From Leuckart and 
Nitsche, after G. O. Sars. 
Anterior end of heart (aorta). 
Ovary. 
Alimentary canal, 
Hepatic diverticula. 
Nervous system. 
Base of thoracic limbs with inward projections 
(oostegites) which form the floor of the brood- 
pouch. 
SR go ho 
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 
