M. Lacaze Duthiers on Circulation in the Gasteropoda. 227 



and one which assuredly occupies a very high place in that group, 

 which presents the same arrangement. 



I hope to be able before long to bring out a complete Monograph 

 of the anatomy of Pleurobranchus aurantiacus, in which the exist- 

 ence of an external orifice of the circulatory apparatus shall be put 

 beyond doubt. As the figure which accompanies this Note shows, a 

 very small orifice (b, fig. A, and c, fig. B) with a raised rim is visible 

 above the external genital organs and in front of the principal 

 branchial vein. This orifice, hidden by the contractions of the body, 

 is very conspicuous in the dead animal. On injecting milk or any 

 other liquid by it, the fluid is seen always to enter the heart ; and 

 on slitting up the branchial vein, there is seen within it an aperture 

 (d, fig. B) leading into a little canal which is connected with the 

 external aperture, and is the channel whereby the fluid injected 

 enters the heart. I have varied the method of injection in every 

 possible way, and always with the same result. I cannot conceive 

 that there is any rupture of the parts or any extravasation of the 



EXPLANATION OF THE FIGURES. 



A. Pleurobranchus aurantiacus, seen in a side view. 



a. Heart. 



b. External orifice of the sanguiferous system, placed before the branchiae 



and above the genital organ. 



B. Enlarged view of the heart, branchial vein, &c. 



a. Auricle. 



b. Ventricle. 



c. External opening through which fluid may be injected into the heart. 



d. Branchial vein laid open at this part to show the internal opening of 



the canal which leads from the external orifice c. 



e. Penis. 



/. Part of the branchial vein, unopened. 



