26 BRITISH TUNICATA. 



The ultimate divisions of the organ are so minute that they 

 can only be observed by the aid of the microscope after a 

 portion of the intestinal tube has been removed,, laid open, 

 aud deprived of the mucous membrane, so as to render the 

 tissue as transparent as possible. 



In Axf-iil/ft nientula the dichotomous division of the tube is 

 very obvious, and the enlargements or ampullae at the junc- 

 tion of the branches are greater than usual, and the}' assume 

 a triangular form ; also oval enlargements frequently occur 

 along the branches, which latter uniting- go to form two long 

 slender ducts that pass backwards within the loop of the 

 intestine buried amidst the vesicular substance already de- 

 scribed, and at length open through the left wall of the 

 stomach about midway between the cardia and pylorus, 

 towards the anterior margin. These two ducts come from 

 the middle portion of the intestine; another duct, passing 

 from the lower part of the intestinal tube, unites with one of 

 those first mentioned just before it sinks into the wall of the 

 stomach. All the three ducts are exceedingly slender; and 

 for their detection it is necessary to dissect carefully the 

 vesicular matter within which they lie buried : when thus 

 exposed their white walls can easily be traced, with the aid 

 of a good lens, running amidst the comparatively dark sur- 

 rounding tissue. 



In Ascidia sordida and A. scalra the arrangement of the 

 parts of the hepatic organ is similar to that in the above 

 species; but in Corella parallelogramma the minute structure 

 is considerably modified. In this species there is a minute 

 network of anastomosing tubes spread over the intestine, the 

 tubes being divided into systems by the interruption of the 

 anastomoses along certain Hues where the twigs end in blind 

 sacs, which are occasionally a little enlarged and rounded. 

 The main branches leading from the network exhibit a ten- 

 dency to divide dichotomously, and unite to form two slender 

 ducts which pass at once from the intestine to the left side 

 and close to the posterior margin of the stomach, into which they 

 pour the biliary secretion a little in advance of the pylorus. 



In Pelonain there is only one hepatic duct, which is very 

 slender, and passes in a fold of the lining membrane or 

 " inner tunic " of Huxley that extends from the intestine to 

 the right side of the stomach, a little way in advance of the 

 pylorus. Before terminating, it receives a twig- or two from 

 the surface of the stomach ; so that in this genus the liver is 

 apparently not confined to the intestine, but is also spread 

 over a portion of the stomach. The ultimate twigs divide 



