206 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



In the myriapods and larvse the same general plan is continued, the 

 alimentary canal still being a simple tube passing from one extremity 

 of the body to the other, with an oral orifice and vent, but in these 

 animals showing a division into gullet, stomach, and intestines (Fig. 62). 

 A difference is also met with according as the animals are carnivorous or 

 vegetable-feeders. In the former the canal is narrow and nearly straight, 

 with a slight dilatation representing the stomach, 

 while in the herbivorous species it is complicated 

 by saccular pouches to delay the onward progress 

 of the food. In the tunicata the division of the 

 alimentary canal into gullet, stomach, and intestines 

 is more marked, since we have in them a distinct 

 oesophagus, stomach, small and large intestines. In 

 the crustaceans there exists a definite digestive 

 apparatus, with the first appearance of distinct 

 glands having for their function the secretion of 

 digestive juices, crustaceans especially having a 

 voluminous liver which secretes a yellowish-green 

 fluid of the nature of bile. In the crustaceans 



-CG 



fW 



OES 



Fig. 59.— Diagram op 

 the Alimentary Ca- 

 nal of an Earth- 

 worm, after Ray 

 Lankester. 



M, month; PH ; pharynx; 

 OES, oesophagus; CG, calcare- 

 ous glands; CP, crop; G, giz- 

 zard ; I, intestine. 



Fig. 60.— Transversa Section op Earth-Worm to Show 

 Position and Relations of the Intestines, after 

 Olaparede. (Jeffrey-Bell.) 



A, cuticle ; B. liypodermis ; C, layer of circnlar muscles ; D. layer of longitudinal 

 muscles ; I, enteric cavity ; M, " green layer ; " N, dorsal vessel ; O, liver. 



the liver has become a symmetrical, lobulated organ, instead of the 

 numerous small follicnli which are found in earlier forms around the 

 alimentary canal, and which pours its secretion into the upper part of 

 the intestine. In the higher crustaceans, such as the crabs and lobsters, 

 there is a short, wide sac, provided with internal hard, calcareous dent- 

 icles, which serves the purpose of a gullet, stomach, and gizzard. The 



