CIRCULATION IN THE DORSIBRANCHIATA. 



259 





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ventral cBSOphageal carries the blood forwards, and the dorsal backwards 

 towards the heart. 



(669.) The independent contractile Fi g- 128 



(ergo circulating) power of each indi- 

 vidual vessel may be very completely 

 proved by an examination of the branchiae 

 of a living Arenicola. A single ramuscule 

 in the branchial tuft may contract and 

 empty itself while the surrounding 

 branches are expanding diastolically. 

 There is no synchronism in the circula- 

 tory movements of these vessels. Both 

 the afferent and efferent vessels of the 

 branchiae are long and tortuous, but 

 discover no cardiac ampulla? in any part 

 of their course. In Arenicola the peri- 

 toneal chamber is filled with a highly 

 corpusculated fluid, the basis of which 

 consists of sea- water, and the presence 

 and movements of which are indispen- 

 sable to the circulation of the blood- 

 proper. By this remarkable mass of 

 fluid, the slender tortuous vessels are 

 shielded from injurious pressure. 



(670.) In Arenicola piscatorum* the 

 generative apparatus consists of six late- 

 ral pouches, which during the months 

 of July and August are in a condition 

 of extreme vascularity. They are quite 

 visible to the naked eye by their bright 

 red colour. The minute structure of 

 these pouches can be studied by carefully 

 dissecting them from their attachments 

 to the abdominal wall of the cavity and 

 placing them under the microscope. One 

 of them thus viewed will be seen to con- 

 sist of a sac or bag, the fundus of which 

 is csecal. The interior cavity throughout 

 the upper three-fourths is a single un- 

 divided space. The attached extremity 

 is formed into two very distinctly marked 

 channels or tubes, the interior of which 

 is formed of cilia that beat in opposite directions. In the glandular 

 tube, the ciliary current sets towards the cavity of the organ ; in the 

 * Dr. Williams, Phil. Trans. 1858, p. 118. 



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Circulation in Arenicola. 



