2/O C. M. CHILD. 



ward current. This groove is probably always open whether the 

 body is distended or not. According to the laws of fluid pres- 

 sure the force of the inward current need be only slight since 

 the pressure it exerts is transferred to every unit of surface and 

 in all directions in the enteron, /. e., the principle by which en- 

 teric pressure is maintained is that of the hydraulic press ; a 

 slight pressure exerted over a very small surface is transferred to 

 a large body of confined fluid and so multiplied many times. 



If these suggestions are correct the passage of water in and 

 out of the body and the process of distension and collapse are 

 accomplished somewhat as follows : When the body is contracted 

 it is probable that the oesophagus is more or less widely open ; 

 observation of living animals or sections of contracted specimens 

 (Fig. i) support this view. This condition of the oesophagus is 

 probably brought about by contraction of the mesenteries which 

 possess a slightly developed transverse musculature. In the 

 contracted state then there is free passage of water in and out. 

 When the muscles relax the cesophageal walls are again brought 

 into more or less close contact, this result being, perhaps, brought 

 about in part by the elasticity of the cesophageal walls or by 

 muscular contraction, and in part by the circulatory currents in 

 the intermesenterial chambers on all sides of the oesophagus 

 (see Figs. I and 2). The approximation of the oesophageal 

 walls decreases the outward currents, while the inward current 

 continues, since the siphonoglyphe remains open. Thus, more 

 water enters than passes out, and the result is of course gradual 

 distension of the body. As the internal pressure increases the 

 walls of the oesophagus are more and more closely appressed 

 and thus the outward currents are decreased still further while 

 the inward current continues undiminished. The increase in 

 pressure and distension will continue until the motive power ex- 

 erted by the cilia of the siphonoglyphe is just sufficient to balance 

 the internal pressure and prevent an outward movement of water, 

 and to replace any loss through outward currents, if such still 

 continue, as perhaps they do to a slight extent. So long as 

 the various conditions remain unchanged distension continues. 



Let us now suppose that a sudden stimulus leads to contrac- 

 tion of the body-muscles. In the first stages of this contraction 



