EXCRETORY SYSTEM OF THE LOBSTER 177 



to the telson, giving off one pair of large arteries in each somite 

 (Fig. 69). 



The vascular system thus consists of an arterial system and a 

 great body cavity, which forms a blood sinus, taking the place of 

 a venous system in other ani- 

 mals. The pressure forcing the 

 blood through the gills comes 

 from the constant addition of 

 blood to the body cavity through 

 muscular heart beats, aided by the 

 vacuum produced when the heart 

 is emptied. Movements of the 

 appendages also tend to keep up 

 a constant circulation in the 

 sinuses. 



THE EXCRETORY SYSTEM. Ex- 

 cretion in the lobster must be 

 comparatively sluggish, for the 

 organs for the purpose are small 

 and poorly placed for active func- 

 tion. This may be due to the fact 

 that the lobster and similar forms 

 are naturally sluggish animals, 

 lying in wait for prey, feeding on 

 carrion, etc., rather than moving 

 about actively in search of food. 

 The nephridia are small flattened 

 coiled organs at the bases of the 

 antennules, and consist of a rather 

 large "bladder" and a small 

 glandular part (Fig. 71). From 

 their characteristic color they are 

 also known as the green glands. 



FIG. 72. The abdominal 

 musculature of the lobster to 

 show the complicated arrange- 

 ment of extensors and flexors. 

 (From Gerstaecker, after Milne- 

 Edwards.) 



The external openings of the nephridial ducts are on the 

 inner faces of the basal segments (Fig. 68, 3). Some excretion 

 of waste matters may also take place through the skin. 



THE MUSCULAR SYSTEM. The muscles of the lobster are 



