212 



THE RISE OF ANIMAL LIFE 



swallowed through the short esophagus and 

 taken into the large cardiac stomach, which 

 functions more as a storehouse for food 

 than as a digestive organ (Fig. 11-11). At 

 the posterior end of this organ is the gastric 

 mill, which is composed of three small 

 tooth-like bodies called ossicles, two in a 

 lateral position and one in the mid-dorsal 

 line. These fit tightly together and the 

 grinding movement is brought about by 

 muscles attached to the outside of the 

 stomach. Before food can pass on into the 

 second stomach, called the pyloric stomach, 

 it must pass through the gastric mill, which 

 grinds all the food fine and renders it 

 digestible. In addition, the two stomachs 

 are separated by filtering "hairs" which act 

 as strainers, allowing only fine particles to 

 pass through. Foods that cannot pass 

 through, such as parts of skeletons, are 

 regurgitated through the mouth. Once the 

 food is in the pyloric stomach some of it 

 passes into the two large multi-lobed di- 

 gestive glands. The enzymes necessary for 

 the complete digestion of the food are 

 secreted by these glands. Digestion occurs 

 for the most part in the upper end of the 

 intestine, although some apparently goes 

 on in the digestive glands themselves. Ab- 

 sorption also takes place here, and in the 

 intestine. All undigested food in the intes- 

 tine passes out through the anus as feces. 



Circulatory system. The circulatory sys- 

 tem of the lobster varies markedly from that 

 of the annelids. Instead of several pairs 

 of hearts, the lobster possesses a single 

 pulsating vesicle that lies on the dorsal 

 part of the body, surrounded by a thin mem- 

 brane and cavity, the pericardium and 

 pericardial cavity, respectively (Fig. 11- 

 12 ) . The heart has three pairs of tiny valved 

 openings, called ostia, which allow the 

 blood to enter from the pericardial cavity. 

 Seven arteries lead away from the heart and 

 convey the blood to all parts of the body. 

 From this point on, the system differs 

 even more radically from that of the earth- 

 worm. The blood leaves the tiny arterioles 



and passes out into spaces, called sinuses, 

 where it bathes the tissues. This type of 

 system is called an open blood system, in 

 contrast to the closed system of the earth- 

 worm. Once the blood leaves the tissues, it 

 seeps into the ventral portion of the cepha- 

 lothorax, then through a set of afferent 

 vessels (to the gills) where gas exchanges 

 take place. It then makes its way to the 

 pericardial chamber of the heart, through 

 efferent vessels (away from the gills), 

 thence through the ostia, and out into the 

 body again. Valves located in the walls of 

 the ostia permit the blood to pass through 

 the ostia in one direction only, namely 

 into the heart. 



The blood contains colorless leucocytes 

 as well as a respiratory pigment, hemo- 

 cyanin, which has a slight bluish color when 

 oxygenated and serves the same oxygen- 

 carrying function as the hemoglobin does in 

 other forms. The blood has remarkable 

 clotting properties. If an appendage is re- 

 moved forcibly, there is hardly any notice- 

 able loss of blood, the clot forming almost 

 at once, and filling the large opening. 



Breathing system. When the lateral walls 

 of the carapace are cut away, large feather- 

 like delicate gills are exposed. These are the 

 breathing organs of the animal. While the 

 crayfish lies quietly in running water, 

 the water moves over the gills without any 

 help from the animal. However, when the 

 demand for oxygen is greater or when the 

 oxygen content of the water is low, a spe- 

 cial modification of the second maxilla, the 

 gill bailer ( scaphognathite ) , waves up and 

 down, like a gondolier sculling a boat, 

 causing the water to flow over the gills 

 in a posterior-anterior direction. This can 

 easily be demonstrated by placing a cray- 

 fish in a shallow white pan and allowing 

 a small amount of India ink to be placed 

 near the posterior end. Great clouds of the 

 carbon particles will issue from the anterior 

 end, indicating a flow of water. Since the 

 openings of the excretory organs, which 

 lie at the base of the antennae, are in the 



