172 REPORT 1851. 



tical cells, transparent and devoid of granular contents, and quite dissimilar 

 in character from the true ova. 



lu the genus Sabella, the peritoneal fluid is opalescent and thickly cor- 

 pusculated; it does not change its colour with that of the true blood, since 

 its colour is the same in those species which are distinguished hy green blood 

 as in those of which the blood is red. The bodies in this instance consist 

 of several varieties of cells, some of which are fibre-like, others orbicular, and 

 bearing granules; but in different individuals of the same species they are 

 constant. In the Nereids (fig. 4} these corpuscles are more or less oviform, 

 and the fluid is distinct, but not large in quantity. In the Aphroditacece it 

 is charged with epithelium-like scaly bodies. To this remark the Aphrodita 

 acideata is an exception, for here the fluid bears no visible morphous sub- 

 stances, and seems to depart little from the standard of salt water. In one 

 other respect this aberrant Annelid approaches tlie Asterias ; namely, in the 

 fact that the peritoneal cavity is lined by vibratile epithelium. 



It must not be overlooked, however, that in this Aphrodite the alimentary 

 system exhibits a curious modification when considered in relation to the 

 plan prevalent among the Annelida as a class. The chylous fluid in this 

 case is transferred from the outside (peritoneal cavity) to the interior of the 

 digestive c£ecal processes, from which it is absorbed into the system of the 

 blood-proper; the exception therefore becomes more apparent than real- 

 No example occurs in the whole class in which the real physiological 

 character of this peritoneal fluid becomes so unequivocal as in that of 

 Glycera alba (fig. 5). The general cavity of the body in this beautiful 

 worm is filled with a fluid, bearing in great abundance blood-red flattened 

 oval corpuscles, resembling the blood-corpuscles of the frog. This is 

 the only Annelid in which the bodies of the peritoneal fluid are coloured. 

 The blood-proper in this species is almost devoid of colour, faintly red, and 

 quite incorpuscular. The bases of the feet in this worm, as in many others, 

 are hollow, and the branchial processes are tubular and filled with the peri- 

 toneal fluid, the interior being lined by vibratile epithelium. The branchial 

 process, which is lined within and without by vibratile epithelium, is filled 

 with the peritoneal fluid, the corpuscles of which move under ciliary agency, 

 peripherally along one side, and centrally along the other of the process. 

 The walls of this appendage contain no true blood-vessels ; and there exists 

 no other respiiatory organ. The inference is therefore irresistible, that 

 the peritoneal fluid it is, and not the blood-proper, which in this case is 

 submitted to the influence of the aerating medium ; and the branchial pro- 

 cess exhibits a structure modified with express reference to the efiicient ex- 

 posure of this fluid rather than of the blood. These facts lead by ob- 

 vious induction to the two-fold division of the process and mechanism of 

 respiration in the Annelida, that, first, in which the true blood is submitted 

 directly to the process of aeration ; and that, secondly, in which the 

 peritoneal fluid is the medium which immediately receives the external oxy- 

 gen. The system for the circulation of the blood-proper under the latter 

 circumstances is little developed ; under the former it is more elaborate, and 

 the volume of the peritoneal fluid is proportionately reduced. There 

 is observable, then, both physiologically and anatomically, an inverse relation- 

 ship between these two systems of nutritious fluids. It follows further from 

 the facts, which, in the example of Glycera alba are so easy of demonstration, 

 that if the peritoneal fluid, which is so unquestionably the recipient, be the 

 reservoir for the collection of the external oxygen, and consequently of car- 

 bonic acid from the blood-proper, the interchange of gases between it and 

 the external sea-water could not occur if both were of the same specific gra- 



