THE TUNIC AT A. 519 



function. M. Lacaze-Duthiers 1 terms this sac an " organ of 

 Bojanus ; " but, as he admits, no opening is discoverable : it 

 would probably be more correct, therefore, to regard it as 

 the representative of the glandular part of the organ of Bo- 

 janus. a 



The heart is an elongated sac open at each end, lodged 

 near the stomach, and close to the hinder extremity of the 

 branchial sac. After a certain number of contractions in 

 one direction, it stops and contracts for the same number of 

 times in the opposite direction. The course of the circula- 

 tion is thus reversed with great regularity. The blood is a 

 clear fluid, containing colorless corpuscles. 



Respiration is effected in the walls of the branchial sac 

 through which the blood is driven. The supply of aerated 

 water is kept up by the currents already mentioned, which 

 subserve the ingestion of food, the respiratory process, and 

 the ejection of effete matters, as well as the expulsion of the 

 generative products. The test in which the body is inclosed 

 is sometimes closely adherent to the surface of the ectoderm, 

 but sometimes is united with it only at the oral and atrial 

 apertures, and by prolongations of the body. In consistency 

 it presents every variety, from soft and gelatinous, to dense 

 and hard like cartilage, or tough like fibrous tissue. In some 

 cases the exterior of the test is covered with horny spines, 

 tubercles, or even with regularly-disposed plates ( Chelysoma). 



In texture, the test may present merely a homogeneous 

 matrix, in which cells like connective-tissue corpuscles may 

 be scattered ; or it may resemble cartilage (JPhaUusia) or 

 fibrous tissue. In most cases it is non-vascular ; but, some- 

 times, tubular prolongations of the ectoderm, divided by a 

 median septum and containing blood, enter it at one point, 

 and thence branch out through its substance. 



In the Chevreulias of Lacaze-Duthiers, 3 the test is some- 

 what like a snuff-box with a movable lid. There is no hinge, 

 however, but the substance of the lid is continuous with that 

 of the rest of the test along the line of junction. And the 

 elasticity of this part causes the lid to stand open, unless it is 

 shut by the contraction of two adductor muscles which are 

 attached to it. 



1 "Les A sci dies simples des Cotes de France." ("Archives de Zooloerie 

 experimentale," 1874.) M. Lacaze-Duthiers has obtained murexide by heating 

 this substance with nitric acid. 



2 There is a close resemblance between the cells of which this organ is com- 

 posed and those which constitute the primitive kidney in the Pulmonata. 



3 u Annales des Sciences Naturelles," 1865. 



