AND PHYSIOLOGY OP THE TUKICATA. 313 



port and protection to them, whatever higher function they may 

 have to perform. They may likewise assist the heart in the perform- 

 ance of its work by their resiliency when the mass is gorged 

 with blood ; for it is evident that, when the interstices or blood- 

 channels are filled, the vesicles will be more or less collapsed in 

 proportion to the pressure of the blood-current ; and when the 

 latter changes its direction the reaction will be assisted by their 

 expansion. In our present state of knowledge, however, nothing 

 positive can be said of the uses of this very curious structure. 



The true hepatic organ, as already intimated, lies beneath this 

 vesiciilar mass, and forms a thin coating on the surface of the 

 intestine. In all the examples observed it is composed of delicate 

 tubes, which divide dichotomously, but frequently without much 

 regularity. At the points where the branches are given off, the 

 tubes are usually enlarged, and the twigs terminate in rounded 

 extremities more or less inflated. 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, and deprived of the mucous membrane, so as 

 to render the tissue as transparent as possible. 



In Ascidia mentula the dichotomous division of the tube is 

 very obvious, and the enlargements or ampullae at the junction of 

 the branches are greater than usual, and they 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 described, 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 ex- 

 posed their white walls can easily be traced, with the aid of a 

 good lens, running amidst the comparatively dark surrounding 

 tissue. 



In Ascidia sordida and A. scahra the arrangement of the parts 

 of the hepatic organ is similar to that in the above species ; but 

 in A. parcdlelogramma the minute structure is considerably mo- 

 dified. In this species there is a minute network of anasto- 



