THE ANATOMY. PROSTOMIUM 3 



and no doubt plays an important part as a tactile organ; in the majority of Oligochaeta 

 it is not large, and in a few it seems to be totally absent. This ' prostomium,' ' prae- 

 stomium,' ' buccal lobe,' or ' upper lip,' as it has been variously termed, is often separated 

 by a furrow from the first segment of the body -the buccal segment, of which it is 

 a process. Sometimes the separation is not marked at all, or hardly marked by two 

 lateral furrows which converge towards the middle line but do not meet. The genus 

 Lumbricus (e. g.) and a few species of Acanthodrilus exhibit a very curious condition of the 

 prostomium ; it is separated from the buccal segment by a cross furrow, but from the 

 angles of this arise two longitudinal furrows which end at the posterior extremity of the 

 buccal segment and as it were continue on the prostomium over this segment. The 

 prostomium has in this case the appearance of being a process of the second segment of 

 the body. Friend has recorded in Allolobophora cklorotica (9) a remarkable extension 

 backwards of the prostomium which reaches as far as the end of the fourth segment. 

 This, it should be stated, is not a characteristic of the species in question, but an 

 occasional variation. 



The genus Phreorydes has a prostomium which is rather elongated and is divided 

 into two halves by a cross furrow at about the middle of its length. In this pai-ticular 

 it recalls the Capitellidae. 



An elongated prostomium — longer than in Phreorycten — chsiracterizes Nais lacustris. 

 The names Nais proboscidea, ' die geziingelte Naide,' given to it by various writers, are 

 expressive of that peculiarity. 



The long prostomium of Rhinodrilus gulielrwi and of Trichochaeta hesperidum has 

 a peculiar structure which has not been described elsewhere. 



In the former species, I erroneously stated that the prostomium was altogether 

 absent ; it is however present, but at times retracted, so that it is not at all conspicuous ; 

 at such times it protrudes from the buccal orifice as a slight conical projection. A 

 prostomium of apparently precisely the same character exists in Trichochaeta ; in that 

 genus I have investigated its structure and relations by means of sections. It protrudes 

 in a fashion similar to that of Rhinodrilus from the mouth, and in sections is seen 

 to arise from a slight invagination of the dorsal wall of the buccal cavity just in front 

 of the brain and at a point posterior to the orifices of the first pair of nephiidia. 

 According to Vaillant the prostomium of Rhinodrilus paradoxus is similarly an 

 'extroversion' of the buccal cavity. It is quite possible that in Trichochaeta, when the 

 prostomium is completely everted (necessarily along with the most anterior section of 

 the buccal cavity), it may have the appearance of being merely a process of the buccal 

 segment, and may indeed prove to be morphologically such, and therefore comparable 

 to the prostomium of other worms. In the meantime I am inclined to think that it 



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