GENERAL ORGANOLOGY. 



139 



of a body-cavity is of importance for their structure, 

 as is most beautifully shown in the group of worms. 

 Here two kinds of excretory organs occur, the ivatcr- 

 vascular or protonephridia, and the scguicntal organs or 

 ncpJiridia ; the former is found in the parenchymatous 

 worms, the latter in the worms which have a body-cavity. 

 The water-vessels begin with a closed network of small 



FIG. 66. 



FIG. 66. Segmental organ of an Oligochffte. (From Lang.) tz. ciliated funnel' if is, dissepi- 

 ment ; 'iff 1 , non-glandular, ^ 2 , glandular, part of the canal; eb, terminal vesicle; /, 

 body-wall. 



FIG. 67. Diagram of the primitive kidney of a vertebrate. (From Hatschek.) Dotted lines 

 mark the limits of the segments. A. anal opening ; P, mouth of the duct of the primitive 

 kidney (/''); JVs, ciliated funnel (nephrostome) ; Af, Malpighian bodies of the segmental 

 tubules (Si. 



canals, reminding one of capillaries, uniting into one or more 

 main stems, which usually open in common to the exterior. 

 Just inside the orifice (porus excretorius) there is usually a 

 dilatation forming a sort of urinary bladder, whose violent 

 contraction expels the excretion (Fig. 64). In addition, 



