256 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



communicate by ducts, or ureters, with the exterior. 

 The chief product of these excretory glands is stated 

 to be phosphate of calcium. 



Among the air-breathing Arthropods we find 

 that the excretory organs are outgrowths of the 

 terminal portion of the intestine, which, varying 

 more or less in size and number, extend some way 



into the body cavity ; 

 they are the organs that 

 are known as the Mai- 

 pighian tubes, and 

 uric acid has been repeat- 

 edly proved to be found 

 in them. It is possible, 

 but it is by no means 

 certain, that they are ho- 

 mologous with the rectal 

 excretory organs which 

 we have already found 

 in the Gephyrea and the 

 Rotatoria. They may be 

 completely wanting, as in 

 many of the Aptera and 

 Pycnogonida ; there may 

 be two as in the 'harvest- 

 men (Opilionida) where 

 they are considerably 

 branched ; or four, as in the blowfly, where they are 

 very short; or six, as in the greater number of insects. 

 Sometimes there is a much larger number, the cock- 

 roach having from twenty to thirty, and some Hymen- 

 optera as many as one hundred and fifty. They are 

 sometimes arranged in bundles, and where there is a 

 common duct leading to the exterior, its walls are 

 sometimes provided with muscular tissue, which aids 

 in the expulsion of the contents. 



The Malpighian tubules are often of great size in 



Fig. 105. Respiratory and Renal 

 Organs of Sepia. 



a. Aorta ; v, vena cava ; v*, posterior 

 vcnie cavsB, c, heart ; rf, enlargements 



