SECT. V THE NERVOUS SYSTEM 8i 



body becomes fixed in a larval stage ; the posterior 

 limbs with their ganglia remain quite rudimentary, 

 while the last few segments develop neither limbs 

 nor ganglia. 



Some light is also thrown on the morphology of 

 the sympathetic nervous system, which is particularly 

 well developed in the Malacostraca. The second 

 oesophageal ring formed by the sympathetic nerve is, 

 in fact, the remains of the original Annelidan oeso- 

 phageal ring, after the splitting off of the portion 

 which carried the brain. The present Crustacean 

 oesophageal commissures, together with the ring 

 made by the sympatheticus, formed the original 

 Annelidan oesophageal commissures. 



We also get an interesting insight into the mor- 

 phology of the Crustacean brain. Originally, when 

 still placed in the prostomium, it consisted of the 

 ganglia of the two pairs of eyes, and of whatever 

 other sensory organs may have been on the prosto- 

 mium, and perhaps also of the ganglia of the first 

 antennae. These sensory centres (with the exception 

 of the last, which were probably situated on the 

 oesophageal commissures) were but collections of 

 hypodermal ganglia, as is clear from the fact that the 

 pair of longitudinal muscles which traverse the head 

 dorso-ventrally (see Figs. 12 and 13) pass between 

 the brain-oesophageal commissures (<^^) ; this shows 

 that the ganglia must have had a hypodermal posi- 

 tion, i.e., must have lain between the hypodermis and 

 the musculature. On the migration of the eyes, the 

 optic ganglia would separate from the ganglia which 



G 



