NOTES ON VARIATION IN THE TUNICATA. 357 



absence of minute plication in the wall, the papillae on the 

 internal longitudinal bars, the condition of the transverse 

 vessels, the shape of the meshes ; the shape, size, and 

 arrangement of the stigmata. 



5. The Dorsal Lamina. Its condition, a membrane or 

 languets ; if a membrane, its relative size, and the presence 

 and condition of transverse ribs and marginal teeth or 

 processes ; if languets, their size and shape. 



6. The Tentacles. Simple or compound ; their number, 

 size, and arrangement. 



7. The Dorsal Tubercle. Its relative size, its position, 

 the peri-tubercular area, the shape of the tubercle, the posi- 

 tion of its aperture, and the curving of its horns — if 

 present. 



8. To these characters may be added any noteworthy 

 points in regard to the alimentary canal and the reproductive 

 organs. 



In describing a Compound Ascidian, besides attending to 

 the above-mentioned characters, or most of them, it is 

 necessary to examine the shape of the body of the Ascidio- 

 zooid, the arrangement of the Ascidiozooids in systems (or 

 coenobii), and the arrangement of the systems in the colony 

 (or cormus). 



The test is always liable to be affected by surroundings 

 such as the object to which the Ascidian is attached, still it 

 very frequently affords good specific characters. 



The number of lobes surrounding the branchial and 

 atrial apertures is perhaps the most constant of all the 

 external characters, and yet even it is liable to a certain 

 amount of variation ; for example, it is the rule for the genus 

 Ascidia to have eight branchial lobes and six atrial, but 

 in some species (e. g., Ascidia mentula) the number of 

 branchial lobes may be seven or nine, and the number of 

 atrial lobes five. I have seen a specimen of Ciona intesti- 



