400 Dr. T. Williams on the Mechanism of Aquatic 



The genus Eunice (fig. 4) presents another and different type of 

 branchial vessels. Arranged in a prominent row of bright ves- 

 sels {b,c,d,e,f), standing erect as florid visible combs at the dorsal 

 base of each foot in the body, the branchiae impart to all the spe- 

 cies of this genus a graceful and characteristic appearance. In 

 every species the branchial vessels divide on a uniform plan pecu- 

 liar to this genus. The primary trunk (a) rises vertically along 

 the inner side of the branchia, and detaches from its outer side at 

 regular intervals, straight vessels, which gradually decrease in size 

 from below upwards ; each branch forms a. straight undividing 

 vessel (fig. 5, i,j), ciirving gently upwards and towards the median 

 line ; these branches become in their number distinctive of species. 

 In some of the smaller species inhabiting the British coasts, the 

 branchige are composed only of a single vessel ; this is the case 

 also with the young of the lai'ger species ; in others they vary the 

 single to the number of six or eight. In Eunice gigantea, ac- 

 cording to the figures of Milne-Edwards, the vessels of each 

 branchia amount to thirty-six in number. These vessels, although 

 perfectly naked and unciliatcd, like those of Arenicola, are both 

 less contractile and retractile ; they extend in this genus from 

 the head to the tail, and equal in number the annular segments 

 of the body. In the dorsibranchiate genera, the branchial 

 organs of which are now being described, the true blood circu • 

 lating in its pi'oper vessels is exclusively the seat and subject of 

 oxygenation. 



The fluid of the peritoneal cavity, abundant in quantity and 

 highly organized though it be in the genera under review, does 

 not, at least by means of any external organs, participate in this 

 great function. Judged by such a test, the genera of this grand 

 order of worms should be marshalled under two primary groups, 

 of which one would comprehend those in which the function of 

 breathing devolves exclusively on the true blood, while the other 

 would be characterized by the fact, that the branchia? are so 

 organized as to permit separately or conjointly the exposure of 

 the chylaqueous fluid. When the branchial apparatus is pene- 

 trated by two separate and distinct fluids, coordinate probably 

 in organic properties, the vascular system of the body will be 

 found in general by so much the less developed by how much 

 the chylaqueous fluid supplants the blood-proper in the branchia;. 

 The structure of the branchial organs becomes thus a significant 

 test of the position of any given species in the Annelidan scale 

 — those being entitled to the highest rank of which the respira- 

 tory organs are designed to aerate the true blood, those the lowest 

 in which the chylaqueous fluid alone circulates in the branchiae. 



The subgenera Lysidice, Aglaura and CEnone, of the genus 

 Eunice, are distinguished in the circumstances now defined from 

 all the former genera of the dorsibranchiate order. Naked un- 



