COMPARATIVE ANIMAL MORPHOLOGY AND PHYSIOLOGY. 313 



the large digestive cavity, they are partly taken in amoeba-like 

 by the endoderm cells, and partly reduced to a state of solution 

 by digestive fluids poured out by those cells. The flagella 

 produce currents, which assist in the circulation of food in the 

 cavity, and the rejection of waste by the " mouth." Ascaris, 

 Earthworm, and Leech are far more highly differentiated in this 

 and other respects. The conversion of radial into bilateral 

 symmetry, in the way already indicated, would cause the 

 " mouth " of a hydra-like animal to be pulled out into a long 

 slit. The closure of this, in the middle of its extent, would 

 lead to the formation of two apertures, one at each end, placing 

 the gut in communication with the exterior. 



The mouth and anus of the Earthworm, &c., can be conceived 

 of as derived in this way from the " mouth " of a form like 

 Hydra. The digestive organs of the Earthworm present a 

 number of special contrivances. The pharynx helps to draw 

 in the food, and the gizzard to crush it, while the lining of the 

 alimentary canal is glandular and secretes digestive fluids. The 

 typhlosole increases the absorptive surface. The Leech is 

 specially modified as an ectoparasitic blood-sucker. 



The Fluke is a very much modified worm, adapted for a 

 parasitic life, and the digestive organs are much specialized in 

 accordance with this. The Tapeworm has no gut, and lives 

 by absorption ; but the equally endoparasitic Ascaris retains a 

 well-developed alimentary canal. 



The segments of the Crayfish bear jointed appendages, which, 

 in the anterior part of the body, are adapted for seizing and 

 crushing food, the reduction of which to a fine state of division 

 is completed by the complex gastric mill. The alimentary canal, 

 like that of the Earthworm, exhibits bilateral symmetry, and its 

 absorptive surface is increased by the ridges present in the 

 intestine. Two very large digestive glands are present, which 

 must be regarded, like all such glands, as extremely complicated 

 pouches or diverticula of the alimentary canal, from which, 

 indeed, they grow out, as the study of development shows. 



In the Mussel, although the mouth and anus are in the middle 

 line, the gut is convoluted, as in the higher forms (except 

 Amphioxus), by which absorptive surface is gained, augmented 

 in this case by a typhlosole. The food is procured by the aid of 

 ciliary currents, and digested by the fluid secreted in the diges- 

 tive gland surrounding the stomach. 



