VASCULAE SYSTEM OF TUNICATA. 



405 



to a vessel. One of these two vessels taking a ventral course breaks 

 up into a net-work wkicli is distributed over the branchial lattice- 

 work, whilst the other passes to the intestine and to the generative 

 organs and branches out upon them. The same main-vessel sends 

 also a branch to the mantle and twigs to the wall of the ccelom 

 (body-cavity). The blood distributed in this set of vessels is col- 

 lected again into a longitudinal trunk lying on the dorsal side of 

 the branchial sac, which also receives vessels from the intestine and 

 from the generative organs. Whether these dispositions which have 

 been observed in the solitary Ascidians have a more general distri- 

 bution is not yet ascertained. 



In the Salpas the short, thin- walled heart (Fig. 212, c), generally 

 having the form of a tube, constricted at intervals, is in connection 

 with a large vascular canal (v), which runs along the ventral surface, 

 and also at the opposite end the heart is directly continuous with a 

 large vessel; the latter, in those forms which possess a so-called 

 nucleus (vi), breaks up at once 

 iuto a reticular system, which 

 is distributed in this body, and 

 represents the intestinal ves- 

 sels of the Ascidians. In other 

 Salpa3 (those without nucleus) 

 it appears to divide into many 

 branches, which run towards 

 the dorsal surface and end in 

 a longitudinal canal. This 

 dorsal vessel (v ') is placed in 

 communication with the ven- 

 tral stem by a number of trans- 

 verse canals (c), which freely 

 anastomose with one another. 

 There exists a further direct 

 communication between the anterior portion of the dorsal vessel and 

 the hinder vessel proceeding from the heart, through a number of 

 vessels which run through the gills and break up there. 



The most important peculiarity of the vascular system of the 

 Tunicata is assuredly the existence of the two longitudinal stems 

 which pass along the branchial sac, and which farther on meet on 

 the intestine. 



Let us suppose, starting from the Ascidians, the intestine to be 

 continued in the direction of the long axis of its anterior division, 

 the branchial sac, so that the anus came to be placed at the aboral 

 pole of the body, then we should find the arrangement of the 

 vascular apparatus to be similar to that of many Worms, even in 

 respect of the detail that the branches of both longitudinal trunks 

 are divisible into visceral (to the branchial sac and intestine) and 

 parietal series (to the body-wall). 



The heart belongs to the ventral longitudinal stem. 

 It is in fact a differentiated portion of that stem. In this 



vi c o v 



Fig. 2L2. Circulatory system of Salpa 

 maxima, a Incurrent orifice, b Excurrerit 

 orifice, br Branchial septum, br' Attach- 

 ment of the gills, vi Visceral mass (nu- 

 cleus), c Heart, v Ventral vascular stem. 

 v' Dorsal vascular stem, v" Communicating 

 transverse stems. (The finer ramifications of 

 the vessels are not represented.) (After 

 Milne-Edwards.) 



