356 LIVERPOOL MARINE BIOLOGY COMMITTEE REPORT. 



some few species in former papers,* but the specimens 

 collected during last summer in Liverpool Bay, taken along 

 with those formerly dredged on various parts of the west 

 coast, have supplied me with additional examples, some of 

 which seem worthy of being recorded. The general conclu- 

 sion at which I have arrived is that not one of the characters 

 usually employed in the description of species of Ascidians 

 is constant. They are all liable to more or less variation, 

 and, in the case of some of them, the range of variation is 

 very great. I regard the following as the characters which 

 should be attended to in describing a Simple Ascidian : — 



1. The External Appearance. This is liable to con- 

 siderable variation according to the surroundings. The more 

 important points to mention are — the general shape, the 

 position and extent of the area of attachment, the position 

 and condition of the branchial and atrial apertures, their 

 lobes, the condition of the surface (rough, smooth, etc.), the 

 colour, the length (antero-posterior), the breadth (dorso- 

 ventral), and the thickness (lateral). 



2. The Test. Its consistence (leathery, gelatinous, 

 cartilaginous, etc.), thickness, strength, transparency, colour 

 (on surface and in section), and minute structure. Under 

 the last head may be mentioned the presence and condition 

 of vessels, test-cells, bladder-cells, pigment-cells, etc. 



3. The Mantle. Its relative thickness, its transparency 

 and colour, the condition of its musculature, the condition of 

 the branchial and atrial siphons, and any peculiarities of the 

 body as seen on removal of the test. 



4. The Branchial Sac. Its relative size, its shape, 

 relative thickness of its walls ; the presence of folds, their 

 number, size, and direction; the arrangement of the internal 

 longitudinal bars in relation to the folds, the presence or 



* See especially Journ. Linn. Soc. Lond., Zool., vol. xv, p. 329, and 

 Proc. R. Phys. Soc. Edin., vol. vi, p. 256. 1881. 



