M. Ussow's Zoologico-Emhryological Investigations. 329 



Phallusia mamvirUata) . The well-known ramified, claviform, 

 caecal tubes*, which are met with in tlie second and third 

 layers of the outer mantle, are developed in the embryos of 

 Cynthia and Phallwiia from five cajcal diverticula growing 

 forth from the main venti-al arteiy-vein [Phallusia marnmil- 

 lata)j and afterwards gradually becoming elongated and rami- 

 fying dichotomously. 



The whole system of the tubes thus ramified forming two 

 rings {Cynthia microcosmus) is nothing but the whole system of 

 the capillary blood-vessels of the outer mantle, which is united 

 to the heart by a thick branch growing forth from the main 

 ventral arteiy-vein. At the beginning of ramification, the 

 thicker blood-vessels have three-layered walls, possessing an 

 external lil>rous layer, a middle one consisting of muscular- 

 fibre rings, and an inner one composed of hexagonal epithelial 

 cells. By the contractions of the muscular layer of the vessels 

 and the pulsations of the heart, the blood is driven to the 

 remotest peripheiy of the outer mantle. The walls of the fine 

 capillary vessels and their claviform enlargements consist of a 

 simple epithelial layer. The complete circulation of the blood 

 in the outer mantle is effected in two ways : — 1 . All the 

 vessels, including their enlarged parts, consist of two tubes 

 grown together ; and in these double vessels, if they may be so 

 called, the centrifugal stream of blood moves on one side, and 

 at the same time the centripetal one on the other (as may be 

 observed, for example, in the embryos of Pyrosoma'f). 2. 

 The whole network of blood-vessels of the outer mantle is 

 divided into two parts (Cynthice), viz. a, the remoter portion 

 of the blood-vessels, which ramifies near the periphery of the 

 outer mantle, and b, the portion of the capillary vessels which 

 are distributed not far from the third fibrous layer of the 

 mantle. If the blood is flowing in the former at a given 

 moment in a direction from the heart, it is flowing at the same 

 moment in the other in an opposite direction, or to the heart. 

 The two portions of the capillary blood-vessels just mentioned 

 are united by lateral branches. In correspondence with the 

 regular change in the direction of pulsation in the heart-tube, 

 the course of the blood changes in all the vessels described 

 by me. 



The outer mantle of the Tunicata can be very easily sepa- 

 rated from the inner one, and never coalesces with the epi- 

 thelial cuticular layer of the latter, which, indeed, may be 

 inferred a j^^'iori from the mode of its formation from the 



* Ann. Sci. Nat. tome v. p. 110, Lowig and Kollikerj Bronn, Weich- 

 thiere, Abth. ii. Taf. 3. 



t Quart. Joum. Micr. Sci. 1872, p. 283, pi. xii. fig. 9. 



