Zool.-Vol. I.] BANCROFT— CHELYOSOMA PRODUCTUM. 325 



spirals out into cones with their apices extending inwardly. 

 At the same time the stigmata are often turned edgewise, 

 so that in surface views their presence can hardly be 

 detected. In these older individuals the edges of the outer 

 vessels also grow beyond the edges of the stigmata, so that 

 from the outside the latter cannot be seen, but only an 

 irregular lot of spaces leading into the bases of the cones. 

 In the figure it is seen that each spiral is composed 

 of but one stigma, and that often the same stigma 

 may be coiled at both ends and form two spirals. 

 In the more mature condition there are two and occasion- 

 ally three stigmata to a spiral, but never more nor less, so 

 far as has been observed. (3) The intermediate vessels 

 (?', fig. 12). They are very irregular, thin walled, and con- 

 nect all the other vessels with each other in the most 

 diverse ways. Receiving the blood that has reached the 

 apex of the cone they distribute it to other vessels. One 

 of their most important functions is to bear the connecting 

 ducts which support the internal longitudinal bars. These 

 ducts are often near the place of union of the intermediate 

 vessels with the outer ones («, fig. 12). Among the older 

 individuals these vessels are much more strongly developed 

 and sometimes almost completely mask the spiral vessels 

 and stigmata. They form the most regular transverse ves- 

 sels that the species possesses, usually, but not always, 

 directly uniting adjacent connecting ducts. This regularity 

 is most pronounced near the endostyle and dorsal lamina. 

 (4) The internal longitudinal bars, which are never papil- 

 lated. They are fairly regular over the whole surface of 

 the branchial sac except where it is growing most rapidly, 

 along the endostyle and dorsal lamina. In these regions 

 all the stages in their formation from the smallest papillae to 

 larger ones whose anterior and posterior processes have 

 just united can be seen. The connecting ducts invariably 

 curve so that their inner ends point dorsally, and in the 

 larger ascidians they are often so crowded as to be two or 

 three times as numerous as those in the figure. 



2 Sept. 17, 1898. 



