388 ZOOLOGY. 



Oraer 1. Ascidiacea. — As an exniiiple of (Fig. 3X6") Tuni- 

 cates, we will now study the internal anatomy of BoUenia. 



On examining the test of this Ascidian, which is mounted 

 on a long stalk, the oral or incurrent orifice is seen at the 

 insertion of the stalk, and the atrial or excurrent orifice on 

 the same side near the opposite end. On cutting open the 

 thick test and throwing the flap over to the left, the deli- 

 cate mantle or tunic is disclosed ; it extends a short distance 

 into the stalk or peduncle This thin hyaline mantle is 

 crossed by two sets of narrow raised muscular bands ; the 

 transverse fibres are arranged concentrically to the two ori- 

 fices, so as to close or open them, the longitudinal ones curv- 

 ing outward from the left side. 



Currents of sea-water laden with organic food pass into 

 the oral orifice, which is surrounded by a circle of tentacles 

 pointing inward, and thence into a capacious saccular bran- 

 chial chamber within the mantle, which contracts at the 

 bottom, where the oesophageal opening is situated. The 

 walls of this chamber, which is over an inch long in a good- 

 sized specimen, and gathered into fringed folds, is sieve-like 

 with ciliated perforations (compare Pig. 386" e), making the 

 walls like a lattice-work, the blood coursing through the ves 

 sels passing between the meshes of the sieve-like walls. 



The oesophagus, which lies at the bottom of this branchial 

 chamber, is also situated near the intestine passing over 

 the anal end into the short stomach. The intestine is long, 

 passing up to the insertion of the stalk, where it is held 

 in place by muscular threads extending into the stalk and 

 attached to the mantle ; it then suddenly bends back and 

 passes straight down to the ventj which opens opposite to 

 the atrial orifice ; the end of the intestine is in part revolute 

 and provided with a fringe of about twenty filaments. The 

 liver forms a broad and flat mass of a bright livid green, and 

 consists of three flat lobes each composed of eight or nine 

 lobules, with very short ducts enveloping the inner aspect of 

 the intestine. The ovaries are two yellowish, large and long 

 lobulated masses extending nearly the whole length of the 

 body, while the right one is a little smaller, and situated in 

 the fold of the intestine. The atrium is that region of the 



