HOMARUS AMERICANUS MILNE-EDWARDS 133 



The podobranchias are protected by the epipodites of the same 

 appendage, while all branchiae are inclosed in the branchial 

 cavity formed on each side of the cephalothorax by the branch- 

 iostegites of the carapace. These two cavities are open to the 

 outside along the lower free edge of the carapace. The water 

 is kept in constant motion and driven out of the cavity by the 

 rhythmic action of the "bailer" which has been described above 

 (epipodite of the second maxilla). The gills consist of a central 

 stem and numerous rows of branchial filaments in which the blood 

 is oxidized. 



Nervous system. The nervous system of the lobster is 

 clearly segmented inasmuch as there is typically one pair of 

 ganglia for each somite. The ganglia of each pair are connected 

 with each other by transverse commissures and with the ganglia 

 of the following somite by longitudinal connectives. Thus the 

 nervous system follows the well known "ladder" type of seg- 

 mented invertebrates. But the high development of the lobster 

 is also demonstrated by the deviations from the ideal type. The 

 anterior three pairs of ganglia form together a more or less com- 

 pact brain or supracesophageal mass, situated dorsally to the 

 alimentary canal. The first pair of ganglia supplies nerves to 

 the compound eyes and forms the optic lobes or protocerebrum. 

 The second pair innerves the antennules and forms the deuter- 

 ocerebrum. The third pair innerves the antennae and forms the 

 tritocerebrum. The rest of the nervous chain is ventral to the 

 alimentary canal, and the fourth pair of ganglia which form the 

 anterior-most part of the subcesophageal mass are connected 

 with the tritocerebrum of the brain by two long connectives, 

 forming a ring around the oesophagus. The subcesophageal 

 mass is composed of six paired ganglia belonging to the fourth to 

 ninth somites. Next come five thoracic and five abdominal 

 paired ganglia. The ganglia situated in the sixth abdominal 

 somite represent the fused ganglia of the sixth abdominal and 

 anal somites. The connectives between the twelfth and thir- 

 teenth somites diverge to allow the passage of the sternal artery 



