EXCRETORY SYSTEM. 67 



When present, organ follicular, and empties in intestine. 

 Secretion solid. 



Myriapoda. Kidney composed of long, convoluted tubes 

 generally opening into intestine at anterior part. 



Arachnida. Several caeca empty into intestinal canal, near 

 its termination, a solid urine. 



Insecta. See p. 64. 



VERTEBRATA. Kidneys are two in number, and generally 

 situated immediately beneath the vertebral columns. Com- 

 posed of numerous vascular tufts (Malpighian bodies), each is 

 placed in relation with the termination of a slender tubule 

 lined with mucous membrane, which afterward unites with 

 its fellows to form a single excretory tube (ureter). The 

 Malpighian bodies are always placed toward the periphery of 

 the organ: when the layer thus occupied is well defined, it is 

 called the cortical portion. The uriniferous tubules extend 

 from the region of the Malpighian bodies toward the centre : 

 when this layer is well defined, it is called the medullary por- 

 tion. The urine is excreted in part only from the Malpighian 

 tufts, some of its ingredients being elaborated from a net- 

 work of small veins surrounding the uriniferous tubules. 

 This network results either from division of the return blood 

 of the renal artery, or is received into the kidney from an- 

 other source. From the analogy such an arrangement bears 

 to the portal circulation in the liver, it is known as the renal 

 portal circulation. 



Two types recognized. (1) Organ not separated into cort- 

 ical and medullary portions; tubules may be ciliated, and 

 urine solid ; portal vein received from posterior part of body 

 and lower limbs, when present : includes Pisces, Batrachia, 

 Reptilia, and Aves. (2) Organ separated into cortical and 

 medullary portions ; tubules never ciliated, and urine always 

 fluid; portal vein received from renal capillaries: includes 

 Mammalia. 



Pisces. Kidneys absent in Amphioxus. "With others, long 

 and narrow, without pelves, and placed upon either side of 

 median line, immediately beneath bodies of vertebrae. Ure- 

 ters may unite and form one tube or empty separately into 

 rectum from above. 



