104 COMPARATIVE ANATOMY 



bract ; internally it is produced into a fine canal which runs 

 up to the cephalic shield, and enlarges suddenly to form 

 (2) the urinary canal, a tube of varying diameter, coiled 

 several times, as shown in fig. 23, B. The canal is lined by 

 large, flat epithelial cells, which rest on a basement membrane, 

 and have conspicuous nuclei. (3) The end-sac, a branched, 

 irregularly-shaped structure lying between the central coils of 

 the urinary canal. It is lined by a flat epithelium, and there 

 is reason to suppose that it represents a part of the original 

 ccelom. It will be seen that the larval A pus has another 

 pair of functional excretory organs, opening at the bases of 

 the second antennae. 



The nervous system of Apus consists of a supracesophageal 

 or cerebral ganglion connected by perioesophageal connectives 

 with a ventral nerve-ganglion chain. 



The supracesophageal ganglion is a quadrangular nervous 

 mass situated in front of the oesophagus below and rather in 

 front of the eyes. The perioesophageal connectives uniting it 

 with the ventral nerve chain make a bold sweep upwards as 

 they pass round the oesophagus into the head, and the ganglion 

 itself is tilted upwards and backwards so that its actual upper 

 and anterior face is really the ventral side, its posterior and 

 lower face the dorsal side. A large nerve passes from each of 

 the two anterior corners of the ganglion to the paired eyes, and 

 a pair pf smaller nerves arising from the middle of the anterior 

 edge is distributed to the median nauplius eye. The fibres 

 composing these nerves spring from a large mass of pyriform 

 nerve ganglion cells occupying the anterior moiety of the 

 ganglion. The posterior corners of the cerebral ganglion are 

 continued into the perioesophageal connectives which, sweeping 

 downwards and backwards on either side of the gullet, are 

 united behind the gullet by a pair of postoesophageal transverse 

 commissures. These commissures spring from a distinct 

 ganglionic swelling on either side, and the more anterior of 

 the two sends forward on either side a branch which, uniting 

 with its fellow of the opposite side above the gullet, forms the 

 anterior cesophageal or stomatogastric nerve. The connectives 

 run back to the second post-oral or mandibular ganglion pair, 

 united by two transverse commissures and giving off the nerves 

 supplying the mandibles. From these ganglia also arise the 

 second cesophageal or stomatogastric nerves. The third pair 



