MORPHOLOGY OF ANIMALS 



the terminal portion of the tubule, which is derived as an 

 invagination from the ectoderm, serves as a collecting tube 

 or reservoir (Fig. 19, B). Generally a single pair of these 

 tubules is found in unsegmented animals, such as Mol- 

 lusca and MoUuscoida; this number may be reduced, 

 however, as in the Polyzoa where they are entirely lack- 

 ing, or in certain Gasteropoda where one of them is sup- 

 pressed, or it may be increased as in the case of certain 

 Cephalopoda (Tetrabranchia), where two pairs are pres- 

 ent. In segmented animals, such as annelids, prototrache- 

 ates, and chordates, it is probable that originally one 

 pair existed in every somite, and this is still approximately 

 the case in some of the simpler members of these phyla 

 (Fig. 20), while in higher forms they are limited to certain 

 segments and have disappeared from others (Fig. 2 1 ) . The 

 segmental character of these organs is so characteristic in 

 the phyla last named that they are called segmental organs. 



Kidneys of Chordata; Pronephros. In the Chordata 

 these organs undergo modifications which deserve 

 especial mention. They lie in the dorsal portion of the 

 body-cavity on each side of the notochord. Only in Am- 

 phioxus do they open individually to the exterior ; in other 

 chordates the peripheral ends of the tubules unite on each 

 side of the body into a duct which extends backwards and 

 opens into the cloaca near the anus; this is the segmental 

 duct (Fig. 20, B). This earliest system of segmental tu- 

 bules in chordates is known as the pronephros^ and it ex- 

 tends throughout the entire trunk of the lowest vertebrates 

 (cyclostomes), though in all higher forms it is limited to a 

 few anterior somites just back of the head (hence called 



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