298 



THE ANIMAL KINGDOM 



To digestive ^lanc 



Eye stalk"! Ant. arteries 

 Rostrum 



A 



Greenglaiid 



Mandible-' 



Pericardial 



"Sinus 



rlnte.stine 

 |^_ri3„st. artery 



rAnus 



f; V< /, V'entral art 



Mandibular 

 1 (-.1 n muscle 



btomacn 



-tery 



Cardiac; 

 chamber 



-Lat. tootli 

 rMed. tooth 

 J_::rc~iix Cecum 



E^tc 



iisoT mus 



clcs 



G astro-/ 

 lilh 



orus 



)^ '^Valves 

 E s oph a0us 



Flexor 

 miisdeS 



Obhque 

 muscles 



Figure 16.8. Internal anatomy of the crayfish. A, Digestive, circulatory, reproductive 

 and nervous systems. B, Stomach (enlarged). C, Musculature of the abdomen. (After 

 Howes.) 



pereiopods are attached to infolded lamina of the skeleton which form 

 an internal framework. In the limbs each joint is crossed by a pair of 

 antagonistic muscles (Fig. 16.6). These attach to the side wall of one 

 segment and insert at the base of the next, which may be extended in- 

 ternally to form a lever. The muscles betAveen the "hand" and "thumb" 

 of the pincers claw fill the large hand. The extensor is relatively small, 

 but the flexor that closes the pincers is enormous, inserting on a large 

 flat plate that extends into the hand from the inner side of the base 

 of the thumb. Little force is needed to hold the pincers shut against 

 the effort of the crayfish, but great effort is required to hold it open. 



The digestive system (Fig. 16.8 A) includes an ectodermal foregut 

 and hindgut lined with chitin, and an endodermal midgut. The fore- 

 gut includes a short ascending esophagus and a large stomach over 

 the mouth. The stomach is divisible into anterior cardiac and posterior 

 pyloric portions. The cardiac stomach contains a gastric mill, including 

 one dorsal and two lateral teeth operated by some 13 sets of muscles 

 (Fig. 16.8 B). The pyloric stomach contains several filters formed by 

 bristles that permit only liquids and very small food particles to pass 

 through. The anterior wall of the cardiac stomach may have a pair of 

 large calcareous discs, the gastroliths. These appear and disappear as 

 they play a role in the molting process (p. 328). 



The midgut and hindgut form a straight narrow intestine from 

 stomach to anus. The midgut portion, lying in the thorax, has a short 

 dorsal caecum extending forward over the stomach and a pair of large, 

 yellowish-green digestive glands that open into it by large lateral ducts. 

 As in the molluscs these glands not only secrete digestive enzymes but 

 also serve as regions of absorption. 



As the mill grinds food to a pulp, juices from the digestive glands 



