196- REPORT — 1851. 



a similar situation in relation to the setiferous feet. These organs assume 

 the same character in E. laureata (S.). The elegant worm described by Au- 

 douin and M.-Edwards* is furnished with branchial organs in form of ar- 

 buscles of naiied vessels, after the pattern of those of the dorsibranchiate 

 Annelids now described. Under the genus Amphinome, occurs Plerone tetrae- 

 dra (of Savignj'), in which the breathing organs assume the form of larger 

 florid bunches of naked blood-vessels, situated on the dorsal aspect of the 

 body, each tuft being protected in front by a bundle of strong bristles. These 

 organs assume a still more beautiful form in Chloeia capillata, in which the 

 division of the vessels occurs on the bipinnate principle. 



The genus Eunice presents another and diiferent type of branchial vessels. 

 Arranged in a prominent row of bright vessels, standing erect as minute 

 combs at the dorsal base of each foot in the body, the branchiae impart to all 

 the species of this genus a graceful and characteristic appearance. In every 

 species the branchial vessels divide on a uniform plan peculiar to this genus. 

 The primary trunk rises vertically along the inner side of the branchia, and 

 sends off from its outer side, at intervals, straight vessels, which gradually 

 decrease in size from below upwards : each branch forms a straight undi- 

 viding vessel, curving gently upwards and towards the median line: these 

 branches become in their number ciiaracteristic and distinctive of species. 

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

 composed only of a single vessel ; this is the case also with the young of the 

 larger species ; in others they vary from the single vessel to the number of six 

 and eight. In Eunice gigantea, according to the figures of Milne-Edwards, 

 the vessels of each branchia amount to thirty-six in number. These vessels, 

 although perfectly naked and unciliated, 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 dor- 

 sibranchiate genera, the branchial organs of which have now been described, 

 the true-blood circulating in its proper vessels, has been proved to be exclu- 

 sively the seat and subject of the respiratory process. The fluid of the peri- 

 toneal cavity, abundant in quantity, and highly organized though it be in the 

 genera j^ist reviewed, does not in the least degree 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 (embi'acing 

 the preceding species) would comprehend those in which the function of 

 breathing devolves exclusively on the true-h\oo(\, while the other would be 

 characterized by the fact that the branchiae are organized such as to permit 

 more or less completely the exposure, in conjunction with the blood-proper, 

 of the chyl-aqueous fluid of the visceral cavity, to the influence of the sur- 

 rounding aerating element. It will be seen in the succeeding description, 

 that when the branchial apparatus is penetrated thus by two separate and 

 distinct fluids, coordinate probably in organic properties, the vascular system 

 of the body generally will be found by so much the less developed by how 

 much the peritoneal fluid supplants the blood in the branchiae. 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 respiratory organs are exclusively designed for the expo- 

 sure of the blood-proper to the action of the oxygenating medium, those to 

 the lowest in which the peritoneal fluid alone circulates in the branchiae. 

 The subgenera Lycidice, Aglaura, and (Enone, of the genus Eunice, are 

 distinguished in the circumstances now defined, from all the former genera 

 of the dorsibranchiate order. Naked, unciliated blood-vessels no longer in 

 * See Cuvier, Regne Animal, AnneMis, pi. 8. 



