84 MR. M. M. HARTOG ON THE ANAL 



raentSj sometimes stopping them if excessive ; hence I 

 refrain from noting with illusory exactness the intervals 

 between each respiratory movement. 



It is to be noticed that the rectum contains^ as a rule, 

 liquid only, the bolus of faeces remaining in it but a short 

 time. By endosmose the liquid in the rectum will tend 

 to be at the same condition o£ gaseous saturation as the 

 body fluid around it, kept constantly agitated by the 

 backw^d-and-forward sway of the stomach. During 

 the short interval that the anus is open an approach to 

 gaseous equilibrium with the external water takes place, 

 even despite the very slight movement of the water (shown 

 by the little change of place undergone by suspended 

 indigo or carmine particles) . In the absence of any other 

 suitable respiratory apparatus, no one can hesitate as to 

 the function of the action I have described. 



In the Nauplius larvae of Cyclops and Diaptomus the 

 working is slightly different. The rectum is a sub- 

 spherical muscular sac, which at regular intervals contracts 

 so as to leave a linear cavity (along the long axis of the 

 animal), and immediately dilates, sucking up the water 

 from without. 



An anal respiration, such as that of Cyclops, is found 

 widely among Crustacea — even those which have well- 

 developed gills like Astacus, which is one of the highest 

 forms. It has been demonstrated in Phyllopoda and 

 Cladocera, and is here probably the exclusive mode in 

 Leptodora, as shown by Weismann. That it is therefore 

 primitive, and should be expected to occur in the primitive 

 or at least very generalized group of the Copepoda, is an 

 obvious deduction. Hence I anticipate that the homoeo- 

 morphic Zoea larvae of the Decapoda will prove to have 

 this same mode of respiration. 



If there be any connexion between Rotifers and Nau- 



