46 Elementary Zoology. 



to contain guanin, an excretory product. The gland consists 

 of a coiled tube opening into a terminal sac, which itself, as 

 already stated, opens on to the exterior. A small " end sac " 

 has been described in connection with the excretory organ, 

 which has been regarded as a remnant of coelom ; in this case, 

 therefore, the green gland of Astactts will be comparable to a 

 nephridium of Lumbricus, in so far as it is a glandular tubCj 

 opening at the one hand into the coelom, and at the other on 

 to the exterior. If this structure be not a vestige of coelom 



'veh 



^'^G. 23.— Dissection of anterior end of Crayfish, to show relations of green gland. 

 (After Marschall.) 



V, vesicle of green gland, G. glandular portion ; St, stomach ; man.mus, mandibular muscle. 



there is still no final difficulty in comparing the green gland 

 with a nephridium ; for in an animal in which the ccelum is so 

 largely aborted as it is in Astacus, it might be very well 

 supposed that the nephridial funnel had disappeared also. 



If there be some difficulty in comparing the excretory 

 system of the crayfish with that of the earthworm, there is no 

 difficulty in comparing the nervous systems of the two animals. 

 In the crayfish we have precisely the same plan of central 

 nervous system. There are a pair of supra-oesophageal ganglia 

 just below the rostrum ; from these arise circum-oesophageal 

 commissures, uniting behind the oesophagus to form a chain of 

 ganglia extending through the body. This chain consists of 

 six separate pairs of ganglia in the cephalothoracic region, and 



