338 Introduction to A n imal Morphology. 



eyes. When they are large, as in Phronima, the brain is large, 

 while in eyeless forms it is exceedingly small ; sometimes it 

 is lobed (Oniscus). In the ventral cord of these there is often 

 a considerable degree of fusion of ganglia, which are 10-12 

 in number, 7-13 in Isopods, 9 in Laemodipoda. In De- 

 capods and Stomapods, as the mouth retreats from the eyes, 

 the lateral commissures are often enormously long, and united 

 in front of the hypopharyngeal ganglion by a transverse com- 

 missure.* In Stomapods there is an anterior small epi- 

 pharyngeal brain, followed by the long united commissures, 

 and a large cephalothoracic ganglion, the fused metameric 

 ganglia of the peri- and post-stomial segments, from which 

 the thoracic limbs derive their supply ; then follow three ab- 

 dominal, and six post-abdominal ganglia, of which the last is 

 often large. In Schizopoda there are six cephalothoracic and 

 abdominal, and six post-abdominal ganglia, and in Anomoura 

 the post-abdominal ganglia are fused into one. In crabs all 

 the post-oral ganglia form a hypopharyngeal central mass. 



The antennar, antennular, and optic nerves arise 

 from the brain ; the auditory nerve varies in its origin. 

 In Cirripedes there are two visceral nerves, one from 

 the ventral ganglion, and one from the pharyngeal 

 ring; these unite in a plexus. An azygos gastric 

 nerve also exists from the ventral ganglion, and there 

 is a corresponding nerve in some Branchiopoda. A 

 cardiac nerve and ganglion exist in Limulus ; but 

 these are obscure in most Entomostraca. The number 

 of ganglia is thus variable, the small numbers indi- 

 cating concentration, not simplicity. The nervous 

 system in Cladocera approaches most nearly to that 

 of Vermes. Apus has the largest number of ganglia 

 (two thoracic, eleven abdominal, and about forty-nine 

 post-abdominal). Branchipus and Artemia have 

 thirteen pair. 



* Apus has a similar commissure. 



