INTERNAL ANATOMY. 



57 



The individual groups of cells have an elongated and 

 more or less ovoid shape with the long axis parallel to the 

 long axis of the body. According to Langerhans their 

 surface is ciliated. Two kinds of cells enter into their 

 composition ; namely, large clear dilated cells, which are 

 separated from one another by fine fibre-like cells of 

 extreme tenuity (Fig. 27). In the latter the nucleus of 

 each cell is placed near the free end of the cell, while in 

 the former it lies nearer the 

 base of the cell. Langerhans 

 found highly refringent con- 

 cretions in the dilated cells 

 which he took for excretory 

 products. That these cells 

 have a capacity for excreting 

 waste matters has more re- 

 cently been shown experiment- 

 ally by F. E. WEISS. The atrial 

 epithelium on the pharyngeal Fig. 27. -isolated ceils from renal 



papilla ; the large cells contain con- 

 bai'S has a similar Character cretions indicated by the black bodies. 



r . (After LANGERHANS.) 



to that forming these curious 



renal papillae on the floor of the atrium. The distribution 

 of these papillae in the vicinity of the atriopore is very 

 irregular and variable and without any regard to a sym- 

 metrical disposition. Although they are undoubtedly to 

 be regarded as a species of renal organ, yet they could 

 not be compared to any portion of the excretory system 

 of the higher Vertebrates. 



Another structure, or pair of structures, which has been 

 considered to belong to the category of renal organs must 

 next be referred to. 



This consists of two funnel-shaped diverticula of the 

 atrial cavity lying in the dorsal (subchordal) ccelom in the 



