1 14 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



of the digestive gland, in the lobster. (In the crayfish they are shorter and 

 broader, and the posterior part is single.) The two gonads have a slender con- 

 nection just in front of the heart. In the female an oviduct will be found arising 

 from each ovary in the region of the heart. Trace it downward over the surface 

 of the digestive gland into the third leg. In the male the vas deferens arises 

 from the testis in the same region and extends backward to the fifth leg. Soon 

 after leaving the testis the vas deferens presents a bend, then widens and pro- 

 ceeds straight to the external opening. (The vas deferens of the crayfish is 

 greatly coiled.) Draw in the reproductive system in your lateral view of the 

 lobster. 



d) The digestive system: The conspicuous parts of the digestive system are 

 the large thin-walled stomach and the voluminous digestive gland. The latter , 

 a paired white organ filling the greater part of the cavity of the cephalothorax, 

 is often called a "liver," but as it really combines the functions of both a liver 

 and a pancreas it would be more appropriately designated the hepato -pancreas. 

 Observe the lobes and small tubules of which it is composed. Note shape, size, 

 and extent of the digestive gland and enter it on your drawing with very light 

 lines. Then remove the left one completely, noting as you do so the place where 

 it is attached to the posterior end of the stomach by an hepatic duct. 



The removal of the digestive gland exposes the stomach more fully. Observe 

 by pushing the stomach over to the right the short esophagus connecting it with 

 the mouth. The stomach is divided by a constriction into a large anterior cardiac 

 portion, whose wall contains hard ossicles, and a much smaller posterior pyloric 

 portion. The hepatic ducts open into the pyloric portion just above the rounded 

 processes which project downward from its sides. Fastened to each side of the 

 cardiac portion of the stomach will often be found a large mass of crystals, 

 calcareous in composition, called a gastrolith. See Hegner (p. 201) for its pos- 

 sible function. From the end of the pyloric chamber find the slender intestine 

 and trace it to the anus. It makes a deep ventral bend just behind the stomach 

 and then ascends to a position directly beneath the dorsal abdominal artery. In 

 the sixth abdominal segment it gives off a blind dorsal sac, or caecum; from this 

 point to the anus the intestine is called rectum. Draw in these parts of the diges- 

 tive system on your outline of the lobster. 



Remove the stomach, leaving the esophagus in place. Cut it open, wash it 

 out, and examine the interior. Note the hard pieces, or ossicles, in the walls 

 of the cardiac chamber, the paired lateral and single median teeth, and the pro- 

 jecting processes covered with fine silky hairs in the posterior division of the 

 chamber, blocking the passage into the intestine. The teeth and ossicles are a 

 grinding apparatus, called the gastric mill, operated by the gastric muscles 

 attached to the anterior and posterior ends of the stomach ; the hairy processes, 

 a straining apparatus, preventing the coarser particles from passing into the 

 intestine. Make a drawing showing the interior of the stomach. 



