INTESTINAL RESPIRATION IN ANNELIDS. 819 



In the latest presentation of his " G-unda-theory " (29), Lang does not refer to this 

 point. 



From what has been said it would seem to be neither necessary nor probable that 

 an anus should have developed in order to permit the expulsion of undigested food- 

 remains. In other words, the expulsive function of the posterior part of the alimentary 

 canal is probably secondary ; the anus was developed in some other connection, and its 

 existence has merely been taken advantage of for getting rid of useless solid matters. 



If the anus is not primarily an aperture for expulsion, can it be considered as 

 primarily for ingestion ? Is the inhalant function the original one ? 



(1) As a first point in favour of this suggestion may be mentioned the wide distri- 

 bution of this inhalant function among those aquatic groups which possess an anus. 

 The phenomena of ascending ciliary action and antiperistalsis give the posterior portion 

 of the alimentary tract, as has been seen, a decidedly inhalant as distinguished from an 

 expulsive character in the Polychseta and aquatic Oligochseta ; and a similar function 

 of the intestine is recognised in the Archiannelida and Sipunculoidea, with some 

 probability in the Echiuroidea, in the Echinoidea and Holothuroidea, the Mollusca 

 (anal chamber of the Aplacophora), many Crustacea and Insecta, and even among the 

 Chelonia. Intestinal respiration (in this case by means of swallowed air) is also known 

 to occur in certain fishes. 



(2) Again, as indicating the essentially inhalant character of the posterior aperture 

 of the alimentary tract, may be mentioned the fact that tactile organs (sensory hairs) are 

 in certain genera of aquatic Oligochseta {Slavina, Pristina) almost or quite as numerous 

 and quite as large round the anus as on the prostomium. In these forms also backward 

 progression is as free and natural as forward progression. I have frequently been struck 

 with this while examining these worms ; the backward movement is not a recoil, or 

 avoidance of something in front of the animal's head, but an active, purposeful, and 

 continued locomotory eff"ort, which is employed as often as or oftener than forward 

 progression. In some specimens of Dero 1 have found a condition approaching that 

 of certain Polychaeta previously described {cf. pp. 806, 807). These may also be 

 briefly again referred to in the present connection ; it will be recalled that the intake 

 of water by the anus, the eye-spots and evident tactile sensibility at the posterior end, 

 the continual exploring movements, and the invariable progression with this end in 

 advance, gave the morphologically posterior extremity the character of the physiological 

 head of the animal. 



(3) The facts of ontogeny may be adduced in this connection. If the anus 

 originally subserved only the function of expulsion, — if it is to be considered as essenti- 

 ally centrifugally produced, as a breaking through, for the purpose of getting rid of 

 matter, — it is difficult to see why its formation should be represented ontogenetically 

 by an invagination from without. The idea may be perhaps better expressed as 

 follows : — The anus exhibits in general a striking morphological similarity to the mouth 

 in its mode of formation — it sometimes, indeed, arises in common with the mouth ; and 



