588 



IN VERTEBRA TE MORPHOL OGY. 



the Synaptidae two much-branched structures termed the re- 

 spiratory trees (k). As their name indicates, these structures 

 are supposed to have a respiratory function, but it is possi- 

 ble that they may also aid in excretion, the waste products 

 of metabolism collecting in the cells lining the interior of the 



FIG. 



269. DIAGRAM REPRESENTING THE INTERNAL ANATOMY OF A HOLO- 



THURIAN (after LUDWIG from LEUNIS). 



i' - cloacal opening. 

 k = respiratory trees. 

 I = Cuvieriuu organ. 

 m = dorsal mesentery. 

 n = duct of reproductive organ. 

 o = reproductive organ. 

 p = longitudinal muscles. 



a = tentacles. 



b = calcareous pharyngeal ring. 

 c = hydroccel ring. 

 d stone-canals. 

 e -- Polian vesicle. 

 /, g, h intestine. 

 i cloaca. 



q radiating muscles of the cloaca. 



tubular branches and being carried to the exterior by a 

 de squama tion of the cells. In addition in a small number of 

 forms (Holothuria) there occur upon one side of the cloaca a 

 large number of slender tubes (I), which, at the will of the 

 animal, can be evagiuated so as to project through the anal 

 opening. These constitute the organ of Cuvier, the function 

 of which is not as yet satisfactorily explained. 



