266 ANNELIDA. 



the segments of the body which we have found in the Dorsibranchiate 

 order, the respiratory tufts are all attached to the anterior extremity of 

 the creature, where they form most elegant arborescent appendages, 

 generally tinted with brilliant colours, and exhibiting, when expanded, 

 a spectacle of great beauty. In some species, as in that represented in 

 fig. 131, there is a remarkable provision made for closing the entrance 

 of the tube when the animal retires within its cavity. On each side of 

 the mouth is a fleshy filament resembling a tentacle ; but one of these 

 (sometimes the right and sometimes the left) is found to be considerably 

 prolonged, and expanded into a funnel-shaped operculum that accurately 

 fits the orifice of the shell, and thus forms a kind of door, well adapted 

 to prevent intrusion or annoyance from external enemies. 



(681.) The curious habitation of the Terebella Medusa is constructed by 

 cementing together minute shells and other small bodies (fig. 132). In 

 neither case is there any muscular connexion between the worm and its 

 abode ; so that the creature can be readily drawn out from its residence 

 in order to examine the external appendages belonging to the individual 

 segments of its body. When thus displayed (fig. 133), the modifications 

 conspicuous in the structure of the lateral oars are at once seen to be in 

 relation with their circumscribed movements, and offer a wide contrast 

 to the largely-developed spines, setae, and tentacular cirri met with in 

 the Dorsibranchiata. In the upper part of the body, rudimentary pro- 

 tractile bunches of hairs are still discernible, but so feebly developed 

 that their use must evidently be restricted to the performance of those 

 motions by which the protrusion of the head is effected ; while upon the 

 posterior segments even these are obliterated, the only organs attached 

 to the rings being minute foot-like processes adapted to the same office. 

 The tentacular cirri, which were likewise distributed along the entire 

 length of the Dorsibranchiate order, are here transferred to the head, 

 where they form long and delicate instruments of touch, and, most 

 probably, assist materially in distinguishing and seizing prey. The 

 branchiae, likewise, are no longer met with upon the segments enclosed 

 within the tegumentary tube, but are placed only in the immediate 



in colour, is non-corpusculated. The converse is true of the chylaqueous fluid, which 

 in every instance abounds in regularly and determinately organized floating cells." 

 " In the Annelida the function of respiration is discharged under two remarkably 

 distinct conditions. Under the first, the chylaqueous fluid alone is submitted to this 

 process; under the second, the blood-proper fulfils the office. The mechanical 

 organs subservient to this function under the former are constructed on a plan dia- 

 metrically different from that of those provided under the latter circumstances. In 

 the Annelid, the true blood and chylaqueous fluid, though coexistent in the same 

 organism, constitute two perfectly distinct and independent fluid-systems. There is 

 between them no direct communication of any sort ; they are, physically, very dissi- 

 milar fluids. An order of branchial organs must therefore be recognized in which, 

 in equal or unequal proportions, the chylaqueous fluid and the blood-proper, either 

 in the same or in distinct appendages, participate in the process of respiration." 

 Ann. & Mag. Nat. Hist. 1853, vol. xii. p. 393. 



