ON THE BRITISH ANNELIDA. l^l 



confounded. In Terebella conchilega, a species very closely allied to the 

 former, but inferior in size, the corpuscles of the peritoneal fluid consist 

 of cells of precisely analogous organization. Like the corresponding bodies 

 in Terebella nebulosa, they are regular and uniform in figure, size and struc- 

 ture. Such remarkable uniformity proves incontestably that they are governed 

 by a definite law of parentage, birth and growth ; that they are restricted to 

 determinate dimensions by a singularly invariable principle of increase. In 

 these two species the cephalic cirrhi or tentacles are composed of hollow 

 tubes, filled with the peritoneal fluid ; the movements of this fluid are 

 determined by the muscular walls of the containing cavity ; the tentacles 

 are extended and contracted by the alternate flux and reflux of this fluid 

 in their axes. In the general cavity of the body it is urged to and fro in 

 large waves by muscular agency, and not by ciliary. Mechanically and 

 physiologically, this fluid is immediately essential to the maintenance of life ; 

 mechanically, by preventing contact between the intestine and integument, 

 thus favouring the circulation of the blood-proper; and physiologically, 

 by furnishing the pabulum out of which the latter fluid is perpetually being 

 reinforced. 



In other species this fluid is characterized by equally distinctive peculiari- 

 ties. In Arenicola Piscatorum it is abundant, and highly charged with 

 corpuscles. Towards the month of August it increases in amount by the 

 influx of oviform bodies, but at every season it abounds in corpuscular ele- 

 ments, which consist of compound granular cells, from the circumferences of 

 which digitate and filiform processes project, imparting to the cells an ap- 

 pearance resembling that of spermatozoid bodies ; the oviform corpuscles are 

 spherical, and distinguished at some point of their circumference by a bright, 

 pellucid, nucleated cell. It will be shown in another part of this memoir, 

 that although these bodies severally resemble the sperm- and germ-cells of 

 this Annelid, yet general analogy requires that they be regarded as peculiar 

 to the peritoneal fluid (Plate II. fig. 3). 



In the earth-worm the space between the intestine and integument is almost 

 obliterated, these two cylinders being so closely bound to each other at the 

 interannular bauds. There exists, however, in the chambers between these 

 dissepiments a small quantity of viscid fluid, the morphotic elements of which 

 consist of uniformly figured spherical molecules, bearing a few granules, and 

 in some instances a nucleus. The true ova, which in this familiar Annelid 

 can be proved never to enter the peritoneal cavity, are perfectly distinct from 

 these corpuscles ; these latter being manifestly peculiar to the peritoneal 

 fluid, another datum corroborative of the view which regards this fluid as 

 fitted to discharge functions independent of the reproductive. In the Onone 

 maculata the fluid is thickly charged with minute orbicular particles, all of 

 the same size and figure, and very minute, and bearing no analogy whatever 

 to the ova of the same worm. 



The BorlasicB and Liniadce, of the family Planarice, present a plan of 

 structure which distinguishes them in a very marked manner from other 

 orders of Annelida. The oesophageal intestine, turned upon itself, terminates 

 here not far from the cephalic extremity of the body, after the manner of 

 that of the Sipunculidce. A hollow sacculated organ then proceeds from the 

 base of the proboscis throughout the body as far as the tail. By Quatrefages, 

 Blanchard and Milne-Edwards, this remarkable organ has been mistaken for 

 the ovarian system. It is, however, a true alimentary system, and it is in its 

 cavity in these species that the peritoneal fluid is contained, and not in 

 the space, which in these instances is obliterated, between this organ and the 

 integument, The corpuscular elements consist here of fusiform and ellip- 



