INTESTINAL RESPIRATION IN ANNELIDS. 821 



ventral, on which progression took place, as happens sometimes in Actinozoa at the 

 present day. 



Obviously, since the undivided mouth aperture is primarily for ingestion (and for 

 expulsion only secondarily, as a consequence of previous ingestion), the two resulting 

 apertures will also primarily subserve the same purpose ; that is, the anus will primarily 

 be an inhalant aperture. 



(5) A few collateral facts may, in conclusion, be adduced, which help to confirm the 

 views here set forth. 



(a) Carlgren (11) has published some observations on the ingestion of food by 

 certain Actinozoa. The cilia of the siphonoglyph or siphonoglyphs beat always in an 

 inward direction, causing a corresponding current of water into the alimentary chamber. 

 The entrance of solid food is occasioned both by cilia and by peristalsis of the 

 stomodoeal tube ; in Metridium and Sagartia mainly by ciliary action ; in Cary- 

 ophyllia partly by peristalsis, which helps in the swallowing of large particles; while 

 in Tealia, Actinostola, and Bolocera.^ peristalsis is the main factor. The undigested 

 food-remains are particulate in Metridium and Sagartia, in which species they are got 

 rid of by the action of cilia, which transport them from the stomodseum to the tips of 

 the tentacles ; in Cari/ophyllia the food-remains are brought up to the inner end of the 

 stomodseum by the cilia of the filaments, and are then expelled by contractions of the 

 body ; in Tealia and Actinostola solid masses are expelled by contractions of the body, 

 possibly assisted by peristaltic action of the stomodseum. 



Thus in the Actinozoa the siphonoglyph is continuously engaged in introducing 

 water containing matters, including oxygen, in solution ; while solid food is introduced, 

 and its remains expelled, through and by the agency of the muscular stomodseal tube. 

 And we may perhaps follow out the line of thought indicated above by supposing that, 

 of the two apertures into which the mouth-opening of the Actinozoa becomes divided, 

 that which represents the end of and leads into the siphonoglyph becomes the anus ; 

 here there is always an inwardly directed ciliary current, which introduces water, but 

 no solid food (compare the experiments on Polychaeta in Part II., which show that the 

 animals avoid ingesting solid particles by the anus, or speedily reject them if they are 

 ingested). The other aperture, representing the main opening of the muscular stomo- 

 dseum, becomes the mouth ; here solid food is introduced, and here solid food-remains 

 are expelled ; in the primitive condition food-remains did not travel to the anus. 



(6) The last point is one which has been referred to previously — the occurrence of 

 digestive glands only in connection with the anterior part of the alimentary tract. 

 This is an obvious consequence from the fact that solid matter is introduced only by 

 the mouth ; it is only this portion of the animal's food which requires to be acted on 

 by digestive juices and so reduced to solution. It may be added, also, that jaws, teeth, 

 and powerful muscular mechanisms are found only at the anterior end of the tract ; 

 and this for the same reason — it is only the nutriment which is introduced at the 

 mouth that can require mechanical division. 



