KESPIEATION OF APHEODITACE^. 263 



spine is furnished with a smooth horny sheath (fig. 129, c, a, 6), com- 

 posed of two blades, between which it is lodged ; and these, closing 

 upon the barbs when they are drawn inwards, effectually protect the 

 neighbouring soft parts from laceration. 



(676.) In the Aphrodite we have 

 an additional appendage developed 

 from the upper part of each lateral 

 oar, in the shape of a broad membra- 

 nous scale, which, arching inwards 

 over the back (fig. 130, c), forms with 

 its fellows a series of imbricated 

 plates, or elytra, as they are techni- 

 cally named (fig. 129, A). Each of 

 the elytral scales is formed by a dou- 

 ble membrane, between the laminae 

 of which at certain seasons the eggs Segment of Aphrodite. 



are found to be deposited, a situation evidently adapted to ensure the 

 exposure of the ova to the influence of the surrounding element, and 

 thus to provide for the respiration of the embryo. 



(677.) The Aphroditaceae, indeed, constitute a group of Annelids to 

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

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

 ages exist, either on the dorsum or any other part of the body. Respi- 

 ration is performed on a novel principle, of which no illustration occurs 

 in any other family of worms. In all the Aphroditacea3 the blood is 

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

 aqueous is exaggerated. Although less charged with organic elements 

 than that of other orders, the fluid of the peritoneal cavity in this famiy 

 is unquestionably the exclusive medium through which oxygen is ab- 

 sorbed. The true Aphrodite type of respiration occurs in Aphrodite 

 aculeata (fig. 129, A). In this species, the tale of the real uses of the 

 elytra or scales is plainly told. Supplied with a complex apparatus 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 of 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 membranous partition from 

 that spacious exterior enclosure bounded above by the felt and elytra. 

 The complex and labyrinthic appendages of the appendiculated stomach 

 lie floating in this fluid and in the chambers which divide the roots 

 of the feet. From this relation of contact between the peritoneal fluid 

 and the digestive caeca, which are always filled by a dark-green chyle, 



