246 LECTURE XT. 



closed vessels may be completely emptied of their blood from one end 

 of the filament to the other. This contraction does not take place 

 simultaneously in every part, but undulatorily, the wave motion be- 

 ginning at the extreme fore-end. It is especially to be noted, that, 

 in this variety of appendage, in which the respiratory is only an inci- 

 dental function, there exist no vibratile cilia. These organs in 

 Ophelia coarctata exhibit analogous characters, while they are less 

 numerous and much shorter. 



" The AphroditacecB constitute a group of Annelids to which the 

 term ' dorsibranchiate ' by no means correctly applies ; that is, in the 

 majority of the species embraced in this order, no branchial appen- 

 dages exist, either on the dorsum or any other part of the body. Re- 

 spiration is performed on a novel principle, of which no illustration 

 occurs in any other family of worms. In all Aphroditacece the blood 

 is colourless. The blood-system is in abeyance, while that of the 

 chyle-aqueous is exaggerated. Although less charged with organic 

 elements than that of other orders, the fluid of the peritoneal cavity 

 in this family is unquestionably the exclusive medium through which 

 oxygen is absorbed. The true Aphrodite type of respiration occurs 

 in Aphrodita aculeata. In this species the tale of the real uses of 

 the ' elytra,' or scales, is plainly told. Supplied with a complex ap- 

 paratus of muscles, they exhibit periodical movements of elevation 

 and depression. Overspread by a coating of felt readily permeable to 

 the water, the space beneath the scales during their elevation becomes 

 filled with a large volume q{ filtered water, which during the descent 

 of the scales is forcibly emitted at the posterior end of the body. It 

 is important to remark, that the current thus established laves only 

 the exterior of the dorsal region of the body. It nowhere enters the 

 internal cavities ; the latter are everywhere shut out by a mem- 

 branous partition from that spacious exterior inclosure bounded above 

 by the felt and the elytra. In this species the peritoneal chamber is 

 very capacious, and filled by a fluid which only in a slight degree 

 contains organised particles. The complex and labyrinthic appen- 

 dages of the stomach lie floating in this fluid, and in the chambers 

 which divide the roots of the feet. From this relation of contact be- 

 tween the peritoneal fluid and the digestive ca^ca, which are always 

 filled by a dark rjreen chyle, it is impossible to resist the conclusion 

 that the contained fluid is really a reservoir wherein the oxygen of 

 the external respiratory current, already described, becomes accumu- 

 lated. From the peritoneal fluid the aerating element extends in the 

 direction of the caeca, and imparts to their contents a higher character 

 of organisation. These contents, thus prepared by a sojourn in the 

 C£Eca of the stomach, become the direct pabulum for replenishing the 



