qnently, we can distinguish the apertures both by their 

 position and by the number of their lobes ; and this is a 

 very important matter, since it enables us to recognise 

 the different regions in the body of the Ascidian ; and also 

 because the exact number of lobes is a characteristic of 

 certain families and genera of Ascidians, e.g., Molgula 

 has six branchial and four atrial lobes, while Ascidia 

 has eight branchial and six atrial. Plate I. shows the 

 exact shape of the lobes and the apertures in Ascidia 

 mentula. 



The branchial aperture {Br.) indicates the anterior end 

 of the body, the region which corresponds to the head end 

 of a man, a dog, or a fish ; and, consequently, the opposite 

 part, which is attached to the ston^e, is the posterior end. 

 Then, the atrial aperture {At.) is invariably placed to the 

 dorsal side of the branchial ; hence, that side of the body 

 near which the atrial aperture is situated corresponds to 

 the back of the man, dog, or fish ; while the opposite side 

 is, of course, ventral. 



Now that it is known which is the anterior end, and 

 which is the dorsal surface, or back, it is easy, by placing 

 the Ascidian alongside oneself, and comparing the parts, 

 to determine which is the right-hand side and which is the 

 left. In PI. I. and in PI. II., figs. 1 and 4, it is the right 

 side of the specimens which is shown. The area of attach- 

 ment at the posterior end frequently involves a certain 

 amount of the left side. 



The Test. 



The outside of the Ascidian — all that is visible, unless 

 one looks into the expanded apertures — is formed by a 

 stout, gristly, translucent layer called the Test or Tunic 

 (hence " Tunicata," the name of the group), which 

 is notable amongst animal structures for containing 



