DR. W. A. HERDMAN OST BRITISH TUNICATA. 275 



form ; branchial sac not folded and having internal longitudinal 

 bars. Excluding the genera Chelyosoma and Mhodosoma (Chrev- 

 reulius, Lac.-Duth.), which have not been found in our seas, the 

 family, as represented by British species, corresponds to Ascidia 

 as used by Forbes (' British Mollusca' &c), and includes the three 

 modern genera Ciona, Ascidia, and Corella, the first and third con- 

 taining each several, and Ascidia a large number of species. Ciona 

 and Corella have a series of languets along the dorsal edge of the 

 branchial sac, while Ascidia has a continuous lamina ; Ciona and 

 Ascidia have the stigmata of the branchial sac straight, while those 

 of Corella are curved. In addition to these and some other less 

 important points, the three genera differ in the course and posi- 

 tion of the alimentary canal from the oesophageal opening onwards; 

 in Ciona (woodcut, fig. 1) it extends beyond the branchial sac 

 posteriorly, while in the other two genera it lies alongside the 

 branchial sac, on the right side in Corella (woodcut, fig. 2), and 

 on the left in Ascidia (woodcut, fig. 3). 



Alder first, in 1863 (Ann. & Mag. Nat. Hist. ser. 3, vol. xi. 

 p. 158), pointed out that the intestine in Corella (then Ascidia) 

 parallelogramma, after leaving the stomach, turned posteriorly*, 

 and not anteriorly as in Ascidia. This peculiarity, however, does 

 not affect the relation of the intestine to the haemal system ; in 

 both cases the curve is away from the heart. Hancock, in cha- 

 racterizing Corella (Ann. & Mag. Nat. Hist. ser. 4, vol. vi. p. 362, 

 1870), after describing the course of the alimentary canal, stated 

 that it was very differently disposed from that of Ascidia, and 

 that the heart occupied a different position. The latter part of 

 this statement requires modification, for although the absolute 

 position of the heart is changed, its position relatively to the 

 intestine is not affected, as may be seen in the following 

 diagrams (woodcuts, p. 276). 



Ciona (fig. 1) shows the simplest and probably the typical 

 condition in which the intestinal loop (?) is completely posterior 

 to the branchial sac (br), the oesophagus and stomach descending 

 on the dorsal side, and the intestine ascending (for a time) on 

 the ventral side. Here the heart, being always in connexion 

 with the stomach, is dorsal, and the intestine lies ventrally and 

 anteriorly to it. 



* The branchial aperture is "anterior," and the oral lamina or languets 

 15 dorsal ;" Hancock considered the endostyle dorsal. 



21* 



