ANUS. 



173 



the general laws of physiology. With the 

 exception indeed of the generative organs 

 which are concentrated in a peculiar part of 

 the body, it is easy to observe that each seg- 

 ment of the body is almost the exact repe- 

 tition of all the rest : they all possess the same 

 organs, and, however the total number of rings 

 may vary, there results no change of any im- 

 portance in the general structure of the animal. 

 Now it may be laid down as a law in phy- 

 siology, that a parity of organization neces- 

 sitates a similitude of action; and it results 

 that as in depriving an earthworm of a given 

 number of segments no organ is removed of 

 which it does not still retain the analogue, no 

 function is completely destroyed ; and conse- 

 quently that if such a mutilation should weaken 

 the vital action, it does not change its nature. 

 This holds good for both the segments of the 

 animal : each continues to possess all the 

 organs essential to individual existence, and 

 consequently if their resisting energy be suf- 

 ficiently great, there can be no reason why they 

 should not continue to live independently of 

 one another, and become two distinct worms.* 

 But if the anterior moiety thus becomes a 

 perfect animal, it is probable that this may 

 not happen to the posterior portion, but that 

 the new individual formed by this part will 

 always continue deprived of generative organs. 

 For the anterior moiety retains exclusively the 

 reproductive organs of the original individual, 

 and there is nothing which authorizes the belief 

 that the earthworm possesses the power from 

 being simply mutilated, of reproducing the 

 whole apparatus on any part of the posterior 

 moiety. This, however, is a circumstance which 

 it would be easy to determine. 



From the sketch that we have given of the 

 organization of the annelida, it will be seen 

 that there exists in this branch of zoology many 

 hiatuses. Anatomists, in fact, have hardly 

 paid attention to any but the leech, the earth- 

 worm, and the na'is, and we possess only a 

 vague notion of the internal structure and 

 physiology of the erratic and tubicolar species; 

 their comparative study would form an interest- 

 ing subject of research. 



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 maux, t. i. tab. 7. Dictionnaire des Sciences 

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 Edwards, Recherches pour servir a, 1'histoire natu- 

 relle du littoral de la France, t. ii. Moquin Tandon, 

 Monograph, des Hirudines, 4to. Montp. 1827. 

 Morrem, De lumbrici terrest. hist. nat. 4to. Bruss. 

 1829. Pallas, Miscellanea Zoologica, 4to. Lugd. 

 Bat. 1775. Home, Lectures on Comp. Anat. Duges, 

 Annales des Sc. Nat. t. xv. Thomas, Memo'ires 



four servir a 1'histoire nat. des sangsues, 8vo. Paris, 

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logia Danica, fol. Copenh. 1788-1806. Schweigger, 

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 tum et vitam, 8vo. Halle, 1817. Klein, Descript. 

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 tura lumbrici terrestris, 4to. Regiom. 1820. Clesius, 

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 tersuchung iiber d. Blutigel, 8vo. Berl. 1817. Knolz, 

 Naturhist. Abhandlung uber d. Blutegel, 8vo. Wien. 

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(H. Milne Edwards.) 



ANUS, (in anatomy,) from Anus vel An- 

 nus, a round, a circle, (syn. ostium recti, 

 podex, culus. Gr. TT^XTO?. Fr. anus. Germ. 

 After. Ital. ano.) is a term commonly applied 

 to the lower extremity of the rectum : properly 

 speaking, it is the inferior orifice of the 

 alimentary tube, through which, in the higher 

 orders of animals, the excrementitious portion 

 of the food, as also the excretions from the di- 

 gestive apparatus, are discharged ; for obvious 

 reasons it is endowed with powers to assist in 

 expelling, as also with the faculty of retain- 

 ing these for a considerable time : such oppo- 

 site but important qualities would infer the 

 existence of a somewhat complicated muscular 

 apparatus, more or less under the influence of 

 the will, as also a structure in other respects 

 worthy of attention. 



The presence of an anus indicates a complex 

 system of digestive organs ; hence in many of 

 the inferior or simpler classes of the invertebrate 

 division of animals it is absent, and in many 

 of the superior of this division, as well as in 

 several of the vertebrata, it presents considerable 

 variety as to structure, function, and position. 

 In some of the zoophytes, such as the in- 

 fusory animalcules there is no central digestive 

 cavity, and of course no distinct outlet. In them 

 absorption takes place by imbibition through 

 pores into cells, in a manner somewhat similar 

 to a sponge ; and most probably excretion (if any 

 occurs) takes place through the same orifices. 

 In others of this class, such as the acalephae, 

 where a rudimental cavity appears in the body 

 of the animal, a single orifice admits the food 

 necessary for its support, and the excremen- 

 titious portion (if any) is ejected through the 

 same opening. In the actiniae, also, where a 



