PHYSIOLOGY OF THE INVERTEBRATA. 243 



considerably. In the Protozoa they are represented by the 

 contractile vacuoles, which perform other functions besides 

 that of a renal organ. 



In the Porifera the waste materials of each cell are thrown 

 into the body cavity, and collectively expelled through the 

 exhalent aperture, or osculum ; a somewhat similar mode of 

 excretion is observable in the Ccelenterata. 



The renal organ of the Uchinodermata, represented by the 

 Asteridea, is the five pouches of the pyloric sac (see later in 

 this chapter). Here there is a fusion of digestion and excre- 

 tion. The water- vascular system, which in other forms 

 performs the functions of excretion and respiration, has a 

 locomotor function in the Asteridea. 



In the Cestoidea and allied orders the water-vascular 

 system opens into the blood system on the one hand, and 

 communicates with the exterior on the other. The water- 

 vascular system is in these animals the representative of an 

 excretory organ. 



The segmental organs, or nephridia, of the Annelida are 

 true kidneys, and form a medium of communication between 

 the circulatory apparatus and the environment. 



In the air-breathing Arthropods, as well as in some 

 Cnostacea {Orchestia), the kidneys are represented by the 

 Malpighian tubules — appendages of the intestine. 



The shell glands of the lower Crustacea have a similar 

 function to the segmental organs of the Annelida; conse- 

 quently they are renal organs. 



The organs of Bojanus in the LamellihrancJiiata are more 

 or less complex sacs or tubes, joining the blood system on the 

 one hand with the exterior of the body on the other. They 

 resemble the [segmental organs of the Annelida, and perform 

 a similar, function — namely, the elimination of waste nitro- 

 genous matters from the system. 



The renal organs in the Lamellihranchiata form a paired 

 kidney ; in the Pulmoga-steropoda, represented by Helix, the 

 kidney is unpaired, being a single renal sac ; but in both 



