2i8 ANNULATA. 



^2i\xed. hepalic arteries. Posteriorly the heart gives off a 

 dorsal abdominal backwards, and a sternal artery down- 

 wards which, on reaching the ventral surface, divides 

 into a ventral thoracic forwards and an ahdomijial back- 

 wards. 



All these arteries supply the organs with pure blood, 

 and the impure venous blood accumulates in the cavity of 

 the body whence it passes out to each gill by an afferent 

 branchial. After aeration in the gills, it is collected by 

 efferent branchials and passed by branchio cardiac canals up 

 the sides of the thorax into the pericardial sinus. 



The heart of the lobster is thus systemic^ and the course 

 of the blood is as follows : — 



.Hearr 



Arfenal bod^ cdvil^ 



^pe'-iCdidial 



Sysrem GjHs ^""'^^ 



/ 



The special point to notice is the haemocoelic body- 

 cavity converting the venous system into a number of large 

 sinuses or spaces, the arterial vessels alone having definite 

 walls. 



The nervous system is constructed on essentially the same 

 plan as that of the Annelida^ but there are more concentra- 

 tions of the ganglia. If the lobster were a simple 

 annelid we might expect to find a dorsal brain 

 over the anterior end of the alimentary canal, a ring round 

 it to the ventral surface, and a double nerve-chain to the 

 hind end, with double ganglia in each segment; but in reality 

 matters are rather different. The brain of the lobster has 

 the true brain portion supplying sensory nerves to the eyes, 

 but, in addition, it has the two next pairs of ganglia belonging 

 to the antennules and antennae fused with it. The anten- 

 nules and antennae are really post-oral appendages, but they 

 move forward in development to the adult position in front 



