260 DR M'INTOSH ON SOME POINTS IN THE STRUCTURE OF TUBIFEX. 



modification of chlorophyll. This author, moreover, shows a distinct nucleus 

 in all his figures of the gland-cells of the same worm, and M. Claparede 

 describes a nucleus in those of Lumbricus. So long as the cell is filled with the 

 granules, it would be a very difficult thing to make out a nucleus, and the large 

 number of nucleated cells from other parts which get mixed up with these in 

 the field of the microscope necessitates some caution in observing. As pre- 

 viously stated, I have not succeeded in seeing a nucleus while the granules 

 were within their investment, nor on watching their extrusion has such been 

 observed. The same result was arrived at after a careful scrutiny of highly 

 favourable longitudinal and transverse sections of the alimentary canal of 

 Tubifex, and after manipulation of the fresh specimens with carmine. 



Amongst the sandy mud and Diatomacese in the intestinal canal, are nume- 

 rous examples of an Opalina (Plate IX. fig. 14, a, b, c, d). Some are about ^th 

 of an inch long, and had the various shapes shown in the figures. The species is 

 probably identical with that found in other minute fresh-water annelids. A few 

 had a swollen anterior end covered with fine striae like a Pecten or Lima, while 

 the posterior or caudal portion was filiform. The usual clear granules and 

 vesicles were present. An elongated granular structure like a canal was occa- 

 sionally visible in the central line, but this could not be traced throughout 

 the entire length of the animal. When freed by laceration of the worm, they 

 rush through the surrounding water very actively by aid of their cilia, for it is 

 to be remembered they are but in their native medium, since the intestinal 

 canal is ciliated, and often gives passage to currents of water. Although a 

 little glycerine is added to the water, their cilia continue in active motion, and 

 the contained globules are very distinct. 



In a few specimens minute parasitic Nematode worms (Plate IX. fig. 15) 

 were coiled at the sides of the intestine near its termination. There is a streak 

 at the snout, and some faint central markings along the body. They appear to 

 be undeveloped or partially developed Anguillulidse, numerous examples of 

 which are frequently found in Lumbricus. 



Circulatory System. — The following observations on the circulatory system 

 are in the main confirmatory of the investigations of M. Claparede, who added 

 considerably to the descriptions of M. d'Udekem. In Tubifex rivulorum the 

 course of the blood-vessels is very regular, and I have met with very few 

 abnormal arrangements. There is a large contractile dorsal vessel which is 

 thrown in the usual state of the animal into many zig-zags or figure of eight 

 crossings, from the fibrous contractions at the septa. A ventral vessel less in 

 calibre and paler in colour has a similar direction, but lies on the opposite side 

 of the alimentary canal. The dorsal trunk, in conjunction with the alimentary 

 canal, is covered by the gland-cells previously described, while the ventral is 

 free. The course of the dorsal vessel is as follows : — 



