200 



METAZOAN PHYLA 



When the food is digested it is absorbed through the walls of the 

 alimentary canal into the coelom and is thus distributed to all parts of 

 the body. Elimination probably takes place in part through the rectal 

 caeca but also in another and very curious manner. Ameboid cells, 

 known as amehocytes, lying in the coelomic fluid, pick up particles of 

 waste and make their way out through the walls of the dermohranchiae — 

 literally, skin gills — thus getting outside the body with the waste, and 

 not returning. The dermobranchiae (Fig. 110 A) are formed by out- 

 pocketings of the coelomic wall which thrust through the skeletal pores 

 and protrude on the outside beneath the epidermis. They are filled with 



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Fig. 114. — Bipinnaria larva of Asterias vulgaris. (From Field, in Quar. Jour. Microsc. 

 Sci., vol. 34.) A, ventral view of a larva five weeks old, to show the bilateral symmetry 

 The body bears a number of lobes, and two bands of cilia, one preoral, the other postoral. 

 The latter is not visible where it passes over upon the dorsal surface, but actually is a 

 continuous band. The internal organs are seen through the partly transparent body. 

 X 52. B, latera view of a three- weeks-old larva, enlarged to the same size as ^. X 63. 

 Because of the difference in age there is lack of agreement in certain details, but it is hoped 

 the two views will enable the reader to visualize the larva. 



coelomic fluid, and their function is chiefly respiratory. When the animal 

 is exposed by the ebbing of the tide they are retracted wdthin the body 

 wall and remain so until the animal is again covered by the return of the 

 tide. The effect of these dermobranchiae, when fully extended, is more 

 or less completely to cover the surface of the body with a soft tissue 

 through which one who touches the animal can feel the firmer wall of 

 the body. The amebocytes are produced in structures attached to the 

 ring canal of the water-vascular system, known as polian vesicles (Fig. 

 Ill) and Tiedemann's bodies. 



229. Nervous System and Behavior. — The nervous system is less 

 highly developed than in the phyla previously studied, there being no 



