396 Dr. T. Williams on the Mechanism of Aquatic 



of the alimentary canal is richly ciliated internally, a singular 

 provision for propelling an incessant current of water from one 

 end of the body to the other. Such a current must necessarily 

 part at once with its dissolved oxygen and its suspended organic 

 particles. The former acts upon the chylaqueous fluid contained 

 in the hollow cylinder embracing the canal. This is artfully 

 accomplished internal respiration ! 



The genus Sabina has been recently constituted by the author 

 to receive several species of tubicolous Annelids which present 

 an organization intermediate between that of Sabella and that of 

 Amphitrite. 



Sabina Poppcea expresses the generic type. The branchiae con- 

 sist of a group of short flexible processes pluming the head ; they 

 support short rudimentary secondary processes, highly ciliated ; 

 each carries a looped blood-vessel. These appendages aerate only 

 the true blood : others, of a quasi-tactile cliaracter, and of un- 

 usually large size, are provided, which assume an occipital situ- 

 ation ; they amount to three in number on either side; they are 

 tubular, non-ciliated, fleshy appendages ; they are penetrated by 

 a large current of corpusculated chylaqueous fluid ; they are ob- 

 viously designed to oxygenize this latter fluid; they are, at the 

 same time, subservient to purposes of touch and defence. 



In Amphitrite vel Sabella alveolata these semi-tactile filaments, 

 similarly situated, are considerably greater in number. Unlike 

 those of the former genus, they are ciliated ; they are hollow tu- 

 bular filaments ; destined to aerate the chylaqueous fluid, they 

 communicate openly with the visceral cavity. In A. alveolata the 

 true-blood branchise are distributed over the dorsal aspect of the 

 body ; they constitute tapering, prominent, blood-red, highly 

 ciliated appendages carrying in their interior axially a single 

 longitudinal blood-vessel, which at the distal extremity returns 

 upon itself. The chylaqueous fluid also penetrates in small 

 quantities into the interior of these processes. By M. Quatre- 

 fages * a complex subdivision of the blood-vessels in these pro- 

 cesses is figured and described. An appearance leading to such 

 an error may be easily produced by pressure. A spirally ar- 

 ranged line of vibratile cilia, coiling from the base to the apex 

 of each appendage, provides for the constant renewal of the 

 aerating medium f. 



In Amphitrite 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 



* " Organisation des Hennellcs," Ann. des Sciences, 3°ie serie, 1848. 



t For illustrations of many of the parts described in the text, the reader 

 is referred to the Report of the Trans, of the Brit. Assoc, for 1851, on the 

 British Annelida. 



