8 



mantle away from the test, so that these layers are no 

 longer in continuity except on the branchial and atrial 

 siphons, and at the posterior end where the vessels enter 

 the test. 



The body-wall (PI. II., fig. 5) is largely formed of 

 connective tissues, both homogeneous and fibrous, with 

 cells, blood sinuses or lacunae, nerves, and the many 

 muscle bundles, large and small, formed of long, fusiform, 

 non-striped fibres. The largest muscle bundles are found 

 about the centre of the right side, where they may be 

 0*5 mm. in thickness. This part in the living Ascidia is 

 often brilliantly pigmented — red, yellow, and opaque 

 white — the coloured cells being exactly like those found 

 in the blood. The connective tissue cells or corpuscles 

 are fusiform, stellate, or amoeboid, and may become pig- 

 mented or vacuolated, like the similar cells of the test. 



At the anterior end the body-wall is prolonged outwards 

 to form the two well-marked siphons, or short wide tubes, 

 which lead in from the branchial and atrial apertures. 

 These are surrounded by strong sphincter muscles (Fl. II., 

 figs. 2, 6, and 7, sph.). Inside the body- wall lies the large 

 cavity called the Atrium, or the peribranchial cavity, which 

 comnmnicates with the exterior through the atrial aperture, 

 and serves to convey away the water which has been used 

 in respiration (see PI. II., figs. 2 and 4,p.hr.). 



The ectodermal lining of the atrial or peribranchial 

 cavity has been called by some French writers the third 

 tunic — tlie first being the test and the second the mantle. 

 The cavity of the atrium is traversed by numerous vascular 

 strands of mesoderm, called connectives (PI. II., fig. 2, 

 con.), passing from the body-wall inwards to the branchial 

 sac. 



Figure G on PI. II. shows the relations of ectoderm (with 

 test over it), mesoderm, and endoderm in a section through 



