PHYLUM ARTHROPODA 



299 



are passed forward through the pyloric stomach so that chemical as well 

 as mechanical digestion takes place in the cardiac stomach. Particles too 

 large to pass through the pyloric filters are regurgitated through the 

 mouth, while the rest filters through into the midgut. Absorption occurs 

 through the linings ot the midgut, dorsal caecum and digestive glands. 



The nervous system (Fig. 16.8 A) is similar to that of the annelids, 

 except that the original brain and the following two ganglia are fused 

 together to form the arthropod brain. During development it arises as 

 three pairs of ganglia, and in the nauplius the third pair are postoral. 

 They later move around the mouth and the three pairs fuse. Circum- 

 esophageal connectives join the brain with the subgastric ganglion, 

 fcrmed by the fusion of the six pairs of ganglia associated with the 

 mouth parts. Beginning with segment 10, bearing the large pincers, 

 each body segment has a bilobed ventral ganglion joined with that in 

 front by nerves to form a ventral cord. .\s in many annelids the cord is 

 paired in the embryo and fused in the adult. This ventral cord has 

 four giant fibers. Stimulation of these fibers produces rapid strong ab- 

 dominal Hexures. Hence, as in the annelids, the giant fibers are asso- 

 ciated with the escape mechanism. 



The circulatory system of arthropods is unique. The coelom, 

 which arises early in development as paired pouches like those of the 

 annelids, later regresses. It is replaced by a system of blood sinuses 

 that appears around the ventral nerve cord and spreads into the space 

 formerly occupied by the coelom. Eventually the sinuses extend through- 

 out the body, even into the limbs and sides of the carapace, forming a 

 hemocoel. 



Arlhrobranchiac 



Inte.stine 



Podobranchia.- 



R ud i ment a ry 

 pleurobranclna" 



Epipodite- 



Gill chamber ■ 



Base of l<z.g 



Pericardial 

 sinus 



rHeart 



■Te.stis 



Sperm 

 duct 



Digestive 

 ola-nd. 



— Artery 

 -Nerve trunk 



Vein from 

 gills 



Vein to gills 



Figure 16.9. Cross section through a crayfish just behind the third pereiopods. 

 (After Howes.) 



