NOTES ON VARIATION IN THE TUNICATA. 



363 



with both absent, with only the dorsal spot, and with only 

 the ventral spot — were all found. The specimens examined 

 were all of moderate size, and, consequently, immaturity 

 could not account for the absence of the pigment in any of 

 the cases. 



The following table shews the number of individuals 

 with each particular variation in the first twenty, the next 

 eighty, and the last fifty specimens examined : — 



Number of 

 specimens 

 examined. 



20 

 80 

 50 



With both 

 pigment 



spots 

 present. 



With, only 



the dorsal 



spot 



present. 



150 



10 



28 

 26 



With only 



the ventral 



spot 



present. 



With 



neither 



of the spotg 



present. 



2 

 14 



7 



64 



12 



23 



5 

 32 

 14 



51 



So far as these numbers go, they shew that about two- 

 thirds of the individuals of Ciona intestinalis have the 

 anterior end of the body pigmented, and more than half of 

 those (about one-third of the whole) have both the pigment 

 spots present. Of the four conditions found, that with both 

 spots and that with neither are the most frequently met 

 with, while specimens with the dorsal pigment spot alone 

 are rarer than those with a ventral spot only. These spots 

 are merely aggregations of round pigmented connective- 

 tissue cells in the mantle. Their function, if they have any 

 definite function, is still unknown. 



Explanation of Plate IX. 



Figs. 1-4. Variations in the internal longitudinal bars and 

 connecting ducts of Simple Ascidians. 

 Variations m the branchial sac of Ciona 

 intestinalis, x 50 diameters. 



Figs. 5 and 6. 



