56 ALFRED GIBBS BOUENE. 



The most anterior septum which is well marked is septum 

 IV . V. I have not been able to satisfy myself as to the presence 

 or absence of any septa anterior to this. The two or three 

 most anterior septa are usually stated to be absent^ but if 

 present they are doubtless very thin and much obscured by 

 the muscular fibres which radiate from the pharyngeal to the 

 body-wall. I am inclined to think, both from dissection of 

 this big worm and from sections of smaller worms^ that these 

 septa are usually present, and that the pharynx really lies in 

 segment i. 



Septa V . VI and vi . vii are very thin. 



Septum VII . viii is also thin. 



Septum VIII . IX is thicker. 



Septa IX . X, X . xi, xi . xii, xii . xiii are very thick. 



The three succeeding septa are less so, and the septa behind 

 these are all very thin. From all the thick septa arise nume- 

 rous muscles, some of which are shown in fig. 3. These pass 

 backwards through one or two succeeding septa to be attached 

 to the body-wall. 



These muscles^ and indeed the thickened septa, must, as has 

 been suggested, give greater strength to the anterior end of 



and the posterior one septum n . N + i. In stating that a certain organ lies in 

 a certain segment great care is necessary with regard to two matters. In the 

 first place, the septum may be attached to the body-wall at some spot more 

 or less widely removed from the intersegmental groove. There is great 

 variation in this matter in dififerent genera ; it is particularly noticeable in 

 Moniligaster. In all such cases which I have examined by means of longi- 

 tudinal sections the muscular fibres of the septum penetrate the body-wall at 

 the right place, but have by a growth of connective tissue become adherent to 

 the body-wall over some region behind the groove. In the second place, 

 certain septa are at times undoubtedly absent or quite rudimentary. Perrier 

 (9) states that two septa are absent in Urochseta. Beddard (3) also states that 

 septum IX . X is almost absent in a species of this genus, and 1 have observed 

 this to be the case in specimens of Urochseta obtained from both South India 

 and Ceylon. The septa viii . ix and ix . x are absent, I believe, in all the 

 species of one genus of Perichsetidse. 



The two or three most anterior septa are, moreover, absent in all worms, 

 or so feebly developed as to have escaped notice, a fact which adds to the 

 difiiculty of enumeration. 



