18 



Guide to Crustacea. 



It is divided into two chambers, a larger one in front, the " cardiac 

 chamber," which serves as a kind of crop, and a smaller " pyloric 

 chamber " behind. In the narrow opening between the two 

 chambers are set three strong teeth which are connected with a 

 system of plates and levers lying in the stomach- wall and moved 

 by special muscles. This development of hard plates and teeth is 

 associated with the fact that the whole stomach is lined by a 

 membrane continuous at the mouth with that which covers the 

 surface of the body and becomes thickened and hardened to form 

 the shell. The external membrane also becomes turned in at the 

 vent to line a considerable part of the intestine. 



On each side of the thoracic region of the body is a large 



O/iening of Liver-duct 

 OfiticNerv^ > Heart 



\_ Brain. li^er •"> , _, ■ , 



Stomach \ \ ' ' Sup,erior wdominaL artery 



fritestine 



Muscles of 

 ' abdomert 



First Can gL ton of 

 Ventral Nerve-Cllaia 



Fig. 4. 

 Dissection of male Lobster, from the side. [Wall-case No. 1.] 



glandular mass, the " liver " or digestive gland, which opens into 

 the alimentary canal by a short duct on each side just behind the 

 stomach. 



The licari lies near the back, just under the hinder part of the 

 carapace. It gives off a number of large arteries in front and 

 behind, as well as one (" descending artery") which runs down- 

 wards to the sternal surface of the thorax. As in other Arthro- 

 poda, there are no distinct veins, but the blood is discharged from 

 the smaller arteries into the general cavity of the body and finds 

 its way by ill-defined venous channels, first to the gills, and from 

 these to the " pericardium " or space surrounding the heart. From 

 the pericardium the blood returns through six valvular openings 

 into the heart itself. 



Tlie cxcrctorji mjaion (corresponding in function with the 



