THE ANATOMY. CLITELLUM 139 



§ 12. Clitellum. 



It is almost, if not quite, certain that the clitellum exists at the period of sexual 

 maturity in all Oligochaeta. Doubtless a good many forms exist which have not 

 been proved to possess a clitelJum, and the absence of this organ has been frequently 

 used by some of the earlier writers as a mark of specific distinction ; but many of 

 these cases have been shown to have be»n inaccurately described — a clitellum being 

 really present in the fully mature worm. The quite recent discovery of the clitellum 

 in the families Aphaneura and Moniligastridae has eliminated any doubt as to 

 the presence of a clitellum in the only families in which it had hitherto eluded 

 discovery. The Moniligastridae, indeed, were placed by Peeeier in a separate 

 group, that of the ' Aclitellians,' supposed to be distinguished by the absence of 

 this highly characteristic organ. The failure to find it in that group was probably 

 due to its temporary presence, or to its not by any means strongly marked 

 appearance. 



There is one genus of Oligochaeta, however, from which the clitellum may 

 conceivably be really absent, or rather represented by a somewhat different 

 structure. This genus is Siphonogaster. The male-pores of Siphonogaster open 

 near to the extremity of long penial processes, which have been referred to on a 

 previous page. The epidermis of these processes has quite the structure of the 

 epidermis of the clitellum, and, in fully mature worms no trace of a clitellum 

 occupying the normal position could be found. Unfortunately, as is the case with 

 practically all the exotic earthworms, we have no such knowledge of the habits of 

 the 'Yoruba worm' as would give a clue to the use of these long 'penial' processes. 

 It is possible that their's may be the function of a clitellum. 



With this possible exception, then, the clitellum appears to be present in all 

 Oligochaeta — in the aquatic no less than in the terrestrial forms. Its presence 

 indicates, with more or less accuracy, the breeding period ; its function is that of 

 producing the cocoon and when that function has ceased the clitellum disappears. 

 The period of existence, however, seems to vary very much in difierent Oligochaeta. 

 The aquatic Oligochaeta without exception, and probably the Moniligastridae also, 

 have a fixed and definite breeding-time, during which alone the clitellum is 

 developed. This period differs according to the species ; in the case of some the 

 autumn, in the case of others the winter, is the time of sexual activity; this 

 restriction of the breeding period accounts for the fact that in so many of the 

 aquatic Oligochaeta the clitellum is unknown. Other influences, too, in addition 

 to the season of the year, are doubtless at work in retarding or accelerating sexual 



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