30 PHYSIOLOGY OF THE INVERTEBRATA. 



In the Paramaecia (Fig. 3, o) the oral regiou or mouth is 

 situated near the anterior end of the body, and an anal 

 aperture is observable in a definite part of the body during 

 the excretion of the undigested portion of the food. There 

 is also an oesophagus which passes into the endosarc,or the 

 semi-fluid portion of the protoplasmic mass. In the endosarc 

 the particles of food give rise to food vacuoles which undergo 

 a rotatory movement round the endosarc ; this movement 

 being caused by the contractility of the ectosarc or " cell-" 

 membrane." During the rotation of the food vacuoles digestion 

 proceeds ; the nutritive portion of the food being absorbed; 

 while the indigestible residuum is ejected through the im- 

 provised anus. As already stated, there is no actual anal 

 aperture ; but there is a very distinctly marked region where 

 the effete matter is ejected. Therefore, in Parami^cwm there 

 is an oral aperture, an oesophagus, a distinct course for the 

 food, though there is no intestine, and an anal region, though 

 no permanent opening. We have in the higher Ciliata a 

 beginning of a true alimentary canal, although of a simple, 

 form ; and even the tract of the moving food vacuoles has 

 been described as " a rudimentary intestinal canal." 



In the Faramcecia there are two contractile vacuoles, 

 situated anteriorly and posteriorly in the ectosarc ; the 

 physiological function of these cavities will be considered 

 later ; but they have probably a dual function — one of these 

 being that of a renal organ.* 



The Poeifeea or Spongida. 



These are animals having many cells, and are the lowest in 

 the zoological scale of the Metazoa. The body cavity (gastro- 

 vascular space) serves alike for digestion and circulation; 

 and it may be remarked that, " with the exception of certain 

 parasites, and the extremely modified males of a few species, 



* The author's paper in the Proceedings of Boyal Society uf Edinlurgh, 

 vol. 16, p. 133. 



