ON THE BKITISH ANNELIDA. 193 



sang vert, S. vesiculosa, S. Unispira, and in Sabina Poppcea*. Like 

 those of the Serpulce, the branchiae in the Sabellce and Sabince subserve the 

 double office of determining an alimentary current towards the mouth, and 

 of aerating the blood. In these genera, the true-blood, in its proper vessels, is 

 the subject of the respiratory change, and not the peritoneal fluid ; the branchiae 

 are organized with this express intention. This is a point of extreme intei-est, 

 to which attention will be drawn under each successive species, as the de- 

 scription proceeds. 



The genus Amphitrite is distinguished from the former by the distribution 

 of the branchiae over the dorsal aspect of the body. To this rule however 

 exceptions occur in some species, as in A.auricoma, in which the branchige 

 constitute comb-like appendages on either side of the third and fourth 

 cephalic rings of the body. In A. alveolata, which expresses the typical 

 structure, the branchial processes are situated on the dorsal surface of the 

 body, except the caudal portion, on which they do not exist. They may be 

 described as tapering, prominent, blood-red appendages (fig. 12, a, a'), carry- 

 ing in their interior, axiallj% a single longitudinal blood-vessel, which at the 

 distal extremity returns upon itself (fig. 12, b, b'). By Quatrefagesf this 

 vessel is figured and described as giving off lateral transverse branches, which 

 envelope the circumference of the appendage : such an arrangement does 

 not exist ; an appearance leading to such an error may be readily produced 

 by pressure. The axis of each process is hollow, and perforated by the fluid 

 of the visceral cavity ; it is along this hollow axis that the blood-vessel pro- 

 ceeds from the attached to the free end of the process (fig. 12, b, b'). So 

 great is the disproportion between the quantity of blood carried by these ves- 

 sels and the volume of the peritoneal fluid which penetrates the process, that 

 in this genus also the respiratory function may be affirmed to be limited almost 

 exclusively to the true-blood. A spirally arranged line of large vibratile cilia, 

 coiling from the base to the apex of each appendage, provides for the con- 

 stant renewal of the aerating medium (fig. 12, a). In Amphitrite, the tenta- 

 cles, grouped into tufts on either side of the mouth, are organized on a plan 

 not dissimilar from that of the branchiae-proper. They consist of fleshy fila- 

 ments, irritable and flexible in the highest degree, hollow on the axis, carrying 

 a single minute longitudinal blood-vessel returning upon itself, and penetrated 

 by the fluid of the visceral cavity. They differ from the branchiae in the im- 

 portant fact of the absence of cilia. In A. auricoma the branchial combs 

 are attached by a single root, expand and divide in a pectinated manner, each 

 tooth carrying only a single longitudinal vessel. 



This species indicates a transition from the typical Amphitrite to the 

 Terebellce. 



In the genus Terebella the branchial organs appear under the form of 

 blood-red tufts, proceeding from three separate root-vessels on either side 

 of the occiput. The vessels divide for the most part dichotomously, forming 

 an arborescent bunch of naked florid blood-vessels ; each ramusculus is 

 enclosed in a delicate cuticular envelope, perfectly destitute of cilia : each 

 ramusculus is also double, that is, it is composed of an aff"erent and efferent 

 vessel. Although extremely transparent and attenuated, the cuticular struc- 

 ture, embracing these branchial blood-vessels, must include some contractile 

 fibres, since each separate ramusculus may be emptied, rendered bloodless, 

 shrivelled, by the compression of the parietes. This provision for reinforcing 



* I have constituted the genus Sabina to receive a new tubicolous Annelid, to be described- 

 in the sequel, which I have placed between the genera Serpula and Sabella, as liaving an 

 intermediate organization. See description of species, part second. 



t Organisation des Hermelles, Ann. des Sciences, 3°»« serie, 1848. 

 1851. o 



