244 PHYSIOLOGY OF THE INVERTEBRATA. 



of these orders of the Mollusca the renal organs communicate 

 by means of an internal opening with the pericardial division 

 of the body cavity. 



Finally, in all the Invertebrata the blood system is not 

 completely separated from the general cavity of the body, 

 and the general cavity of the body has openings placing it in 

 communication with the exterior medium. 



In the Vertebrata the urinary organs consist essentially of 

 a number of coiled tubes, and open to the exterior by special 

 openings, which are usually common to the generative organs. 

 " The individual tubules of which the Vertebrate kidney is 

 composed do not open directly to the exterior, as do the 

 segmental organs of the Annelids, but there is present on 

 each side of the body a duct — a kidney duct — which receives 

 the tubules of its own side, and opens posteriorly into the 

 cloaca. They also possess an important structure peculiar 

 to the kidney of the Vertebrata, known as the Malpighian 

 body, which consists of a capsular widening of the lumen 

 of each tubule, into which projects a coil of arterial blood- 

 vessels known as the glomerulus." 



Besides the excretory or urinary organs, there is another 

 class of secretory apparatus which must be alluded to — we 

 refer to the unicellular and multicellular integumentary organs. 

 These are found largely represented in the Insecta, and they 

 belong to the category of oil and fat glands, whose function 

 is to lubricate the integument and its special coverings. 



" Aggregations of cells whose function is to secrete cal- 

 careous matters and pigments * are especially widely present 

 in the integument of the Mollusca, and serve for the building 

 up of the beautifully coloured and variously shaped shells of 

 these animals." 



* Dr. MacMmmhas thown that whenever hsemochromogen is present in 

 the fluids and organs of the body, it has been produced by excretion ; in 

 other words, it is an excretory pigment. But there is one exception— 

 namely, in a beetle {Stapliijlinus olens), as its testes seem to contain this 

 substance with liEemoglobin. 



