ON THE BRITISH ANNELIDA. 191 



the peritoneal cavity. One or two exceptions only can be urged against the 

 statement that all Annelida breathe either by an external or an internal me- 

 chanism ; in the former case special organs are nearly always provided, in the 

 latter never. All the external branchial appendages are again subdivisible 

 into two leading varieties, radically and essentially distinguishable. In one, 

 the branchial organ is constructed with special reference to the exposure of 

 the blood-proper to the agency of the respiratory element ; in the other, the 

 branchia is a mere hollow process filled with the chyl-aqueous fluid of the 

 peritoneal cavity. Without this division, which is now for the first time sub- 

 mitted to the consideration of the physiologist, no correct ideas could have 

 been formed with refei'ence to the nature or the mechanism of the process of 

 respiration in those genera of Annelids in which the true-blood, in proper 

 blood-vessels, is not brought directly under the action of the surrounding 

 water. Neither is it possible, without the new light afforded by this theory, 

 to comprehend the manner in which the function of breathing is discharged 

 in the Entozoa, in which the integuments are perfectly devoid of proper 

 blood-vessels. 



The blood-proper in the external branchia of the Annelida is distributed 

 on two distinct plans. According to one method, & plexus of blood-vessels 

 embraces the circumference of the branchial process (fig. a, a, a, a) ; while 

 under the other type, the axis of the appendages is traversed longitudinally 

 by a single blood-vessel, which at the extreme end returns upon itself. The 

 NereidtB present examples of the former type, the genera Spio, Cirrhaiulus, 

 Eunice, &c., of the latter (figs. 16, 18). 



The body of the Annelid is for the most part vermiform in figure ; it is 

 generally cylindrical in outline, but frequently flattened, or more or less oval. 

 It is composed of a longitudinal succession of annuli or rings, which first 

 suggested, as already stated, the name of the class to the mind of Lamarck. 

 In structure these rings are neither horny nor calcareous ; they are always 

 fleshy and soft. The true Annelid is distinguished therefore from the true 

 articulated animal in the perfect absence of any approach to a hard skeleton. 

 The segmentations are divided from each other only by a circular band of 

 muscular fibres ; the annular segments are not, as in the Articulata, perfectly 

 distinct from each other ; the longitudinal muscles pass over and under the 

 constricting circular bands. The segmentation therefore is not real, it is only 

 apparent. The rings form the bases of the appendages ; the latter grow out 

 of the former. The structure of each is produced laterally under various 

 shapes to constitute the foot. The feet are never situated perfectly dorsally 

 or perfectly ventrally, always more or less laterally. Each appendage, that 

 is, the lateral processes of each segment, is divisible from above downwards 

 into a dorsal and ventral half. The dorsal group of appendages is called the 

 superior or dorsal foot, comprehending a cirrus, which may be flat or oar- 

 shaped, tapering or cylindriform ; a true branchia, which in shape may be a 

 tapering lamelliform naked vascular, or cylindriform and ciliated process; 

 and lastly spines, which are imbedded in the central substance of the foot for 

 the purposes of mechanical support, and setse or bristles, of numerously 

 varied forms, for the purposes of locomotion or tube-making. In each foot 

 then there is discernible, — 1st, abranchial organ, which is generally developed 

 on the dorsal moiety ; 2nd, a tactile process or cirri, which are for the most 

 part of largest size in the superior foot ; and 3rd, the setae and spines, which 

 are constant in shape, although diff'ering at the anterior, middle, and posterior 

 thirds of the body. The annular segments are most distinctly marked in the 

 middle third of the body, least so at the tail. In many genera the cirri are 



