DP M'INTOSH ON SOME POINTS IN THE STRUCTURE OF TUBIFEX. 255 



absence of the bristles and other points, in which he shows the longitudinal 

 muscular coat separated into six divisions, viz., — two dorsal, two lateral, and 

 two ventral. I have not been able to see this arrangement in the transverse 

 sections of the minute forms of Tubifex, and even the separation at the bristle- 

 sacs is comparatively indistinct. This ambiguity is no doubt due to the small 

 size of the specimens. In the living animals, however, certain rows of papillae 

 may occasionally be observed, which probably mark the dorsal and ventral lines. 

 The circular muscular coat is much thinner than the longitudinal. The latter 

 forms in transverse section numerous well-defined fascicles, and in the fresh 

 condition these are bounded internally by a membranous layer with many cells. 

 At the period of reproductive activity the neighbourhood of the eleventh seg- 

 ment becomes almost opaque from a cellular covering. This appears to be due 

 to an increased development of the cellulo-granular elements of the chorion. 



Adhering by short stalks to the walls of the body posteriorly in both forms, 

 were numerous parasitic vorticellse (Plate IX. figs. 4 and 5), having an active 

 crown of cilia, and numerous globules and granules in their interior. In some 

 examples these were very numerous, often in groups of three or four, but they 

 rarely occurred on the terminal segment, except in decomposing individuals. 

 In contraction the base becomes finely corrugated. The free motion of the tail 

 of the worm in the water places these organisms under very favourable condi- 

 tions for aeration and food. Fungi, also, may frequently be seen growing on the 

 disorganised anterior segments, while the posterior are in full activity. Fresh water 

 annelids, indeed, are prone to have such growths, just as young salmon are 

 under similar circumstances. The anterior part of the body becomes first 

 attacked, dissolving into a granular mass swarming with fungi and infusoria. 

 The segment immediately behind the decayed portion shows its integument 

 corrugated and thrown into transverse rugae, while the perivisceral corpuscles 

 and the blood have disappeared. The next septum is strongly contracted, and 

 in marked contrast to the succeeding segment, within which are many perivisceral 

 corpuscles of the ordinary appearance, and whose dorsal vessel pulsates 

 vigorously. 



Perivisceral Fluid and Corpuscles. — The perivisceral fluid occupies, as usual, 

 the space within the body- wall all round, from the tip of the snout to the tail, sur- 

 rounding and bathing the digestive and other structures contained therein. In 

 Tubifex rimdorum, the perivisceral corpuscles (Plate IX. fig. 6, a), are very con- 

 spicuous, and differ characteristically from those of the succeeding form from 

 the lakes* They are rounded bodies filled with circular granules of consider- 



* Whatever result more extended investigations may give with regard to the specific value of 

 characters derived from the shape of these bodies, it is right that the name of Dr Thomas "Williams 

 should be honourably remembered in conjunction therewith. Vide his paper on the " Blood-proper 

 and Chylaqueous Fluid of Invertebrate Animals," Philos. Trans. P. II., 1852. 



