AXATOMY AND rHYSIOLOGY. '27 



dichotomously with considerable regularity and terminate in 

 round or ovate vesicles, which are very numerous and form a 

 distinct, opaque, yellowish layer. 



The liver in Styela is not more conspicuous than it is in 

 Avidia. It is well developed, nevertheless, and is provided 

 with its secreting vesicles and ducts. In S. tulierosa, and, 

 indeed, in all the members of this genus that have come under 

 my observation, there is a fold of the lining membrane 

 within the loop of the alimentary tube, which passes between 

 the stomach and intestine. This fold is united to the pyloric 

 end of the stomach, where there is a caecal prolongation of 

 that organ. The hepatic ducts lie within this fold ; and before 

 they reach the stomach, in this species, they unite to form 

 a simple, slender duct, which opens into the left side of the 

 caecum. The branches of the ducts ramify dk-hotomously 

 over the lower portion of the intestine, and communicate with 

 comparatively large rounded vesicles arranged like those in 

 Pelonaia. 



In Chii-rliita there is only one hepatic duct, which passes 

 from the middle portion of the intestine and opens into the 

 alimentary tube immediately below the rounded stomach. 

 The branches of the duct ramify over the intestine, dividing 

 dichotomously, and ending in comparatively large, oval 

 vesicles. Exactly the same form of organ is observed in 

 1't'i-njiJiom but in this genus the duct opens through the 

 right wall of the stomach, near the pylorus. The hepatic 

 organ in this interesting form was undoubtedly noticed by 

 Dr. Lister; for he figures and describes, in his well-known 

 memoir in the ' Philosophical Transactions/* " transparent 

 vessels" ramifying over the intestine; but he does not appear 

 to have observed the terminal vesicles, and the termination of 

 the duct in the stomach, or he scarcely could have supposed, 

 as he did, that the vessels he described were lacteals. 



With this exception, this peculiar form of the hepatic- 

 organ seems entirely to have escaped notice until A. Krohn 

 gave a very good description of a similar structure in a paper 

 "On the Development of the Ascidians," published in Midler's 

 ' Archiv,' 1852-53t. The species examined by this naturalist 



* " Some Observations on the Structure and Functions of Tubular and 

 Cellular Polypi and of Ascidia?/' ' Phil. Trans./ 1834, p. 380. 



f See ' Scientific Memoirs,' edited by Henfrey and Huxley, p. 328. Before 

 I was aware of the discovery by Krohn, I had worked out the details of the 

 hepatic organ in the genera mentioned in the text; it was therefore highly 

 satisfactory to find his description of this organ in A. mamillata agree so 

 closely with my observations, particularly in A. mentula. 



