152 GENERAL ZOOLOGY 



ing, perform the operation of grinding. The posterior part 

 of the stomach is filled with slender filaments which extend 

 from the wall out into the cavity. These filaments prevent 

 the unbroken particles of food from passing through into 

 the intestine, allowing only the thoroughly ground-up food 

 to do so. Absorption does not occur in either division of the 

 stomach but in the intestine, which extends straight from 

 the stomach to the ventral surface of the telson. A pair of 

 digestive glands lie one on either side of the stomach and in- 

 testine. They open by tubes into the posterior division of 

 the stomach and have the combined functions of digesting 

 food and absorbing some of the products of digestion. 



The Circulatory, Respiratory, and Excretory Systems. The 

 heart is a muscular organ lying beneath the dorsal body 

 wall posterior to the stomach. The blood finds its way into 

 the heart through three pairs of openings furnished with 

 valves to prevent the escape of blood. When the heart con- 

 tracts the blood flows both forward and backward at the 

 same time through the tubes called arteries. Five arteries 

 pass forward to the organs in the front part of the body 

 while two are directed backward to supply blood for the 

 posterior region. These arteries branch into many smaller 

 vessels with open ends. From the open ends the blood 

 flows into spaces called sinuses surrounding the internal 

 organs. of the body. All the sinuses connect with a still 

 larger, median cavity, the ventral sinus, lying along the 

 ventral wall of the thorax and abdomen. Branches from 

 the median ventral sinus extend out into the gills. The 

 blood is carried back to the base of the gills by another set 

 of channels parallel to the set entering the gills. Finally 

 the blood reaches the pericardial sinus, in which the heart 

 lies. The delicate plume-like gills (Fig. 85) are attached 

 to or near the basal joints of the thoracic appendages. On 

 each side of the body they extend dorsally into partially 



