Gatty Marine Laboratory ^ St. Andrews. 249 



The first appearance of the septum in the sections is 

 heralded by a tuft of muscular fibres attached to the exterior 

 of the j;ut-\vall and the ventral septum ; then the muscular 

 ring (tlie ventral septum and the gut l)eing free) loosely en- 

 velops the gut and its vessels, besides the ventral blood-vessel 

 and its mesentery, almost to the ventral longitudinal muscles, 

 but leaves the dorsal blood-vessel for the most part free. 

 The septum, indeed, springs on each side from the lower 

 wall of the vessel, and encloses that part of the mesentery 

 between the vessel and the sinus around the gut, whilst the 

 distal part of the mesentery passes freely to the gap between 

 the dorsal longitudinal muscles. This muscular sheath or 

 tubular chamber by-and-by swells out into a large area, its 

 upper arch or roof being attached on each side to the body- 

 wall between the dorsal and ventral longitudinal muscles, 

 and its floor stretches from the median ventral mesentery 

 to the wall of the body a considerable distance below the 

 attachment of the roof. This chamber encloses the alimentary 

 apparatus and the gonads, but the more advanced sperms 

 lie in the two coelomic chambers outside its roof — that is, 

 between the latter and the body-wall (PI. IX. fig. 15). It 

 is difficult to explain the exact nature of these septa — 

 whether they are modifications of the ordinary septa, which 

 extend far backward in the caudal region, or only the 

 ordinary septa sliced so as to present these characteristic 

 appearances, — for it is unlikely that two septa would fall 

 into the line of section. These septa seem to differ in 

 disposition and aspect from those in front, and are probably 

 associated with the special functions of the caudal region — 

 respiratory, purely intestinal^ or otherwise. 



On viewing the animal externally from the dorsum, a 

 broad fillet passes from each side of the collar anteriorly and 

 slopes obliquely inward and backward on the dorsum to the 

 constriction behind the third bristle-tuft, then bends a little 

 outward, and is continued along the dorso-lateral . region 

 posteriorly. A groove exists at the collar just below the 

 anterior end, and which apparently is functional also for the 

 median ventral ridge and groove, so that, if ciliated, it may 

 send a current outward and forward to it. These ridges 

 apparently are those which show the remarkable pennate 

 arrangements in the hvpoderm in the preparations (PI. IX. 

 fig. 20). 



In certain longitudinal sections (PI. VIII. fig. 18) the dark 

 pigment stretches as a broad band behind the collar, a gap 

 intervening between it and the edge of the fold behind, such 

 probably representing a sensory groove, and its borders have 



Ann. li- Mag. X llt'st. Ser. 8. Vol. xix. 17 



