172 MERISTIC VARIATION. [part I. 



*135. Ascidia plebeia (Alder): branchial sac has very characteristic 

 appearance and is very constant in the size of meshes, papilla? &c. 

 One point is liable to variation : as a rule the transverse vessels are of 

 the same calibre, but in several specimens every fourth vessel is much 

 wider than the intervening three. Herdman, p. 331. 



Stigmata and Meshes. 



136. Ciona intestinalis : meshes vary but according to no apparent 

 method: 5 stigmata in a mesh normal; 4 and 6 met with frequently; 

 10 the utmost seen. Herdman, p. 332. 



137. Ascidia aspersa. In typical specimens, transverse vessels all same 

 size, the meshes being square and undivided, but individuals occur in 

 which many (not all) of these square meshes are divided by delicate 

 transverse vessels into pairs of oblong areas. Herdmast, p. 332. 



138. Styela grossularia. The genus Styela is characterized by the 

 presence of branchial folds, normally four on each side, but in this 

 species the folds are almost obsolete, being entirely wanting on the 

 left side and reduced to a single slight inward bulging on the right 

 side, bearing internal longitudinal bars. This fold is separated from 

 the dorsal lamina by a broad space without internal longitudinal bars. 

 A similar wide space is present on the left side of the dorsal lamina, 

 and two others on the vertebral edge of the sac, one on each side of the 

 endostyle. These spaces vary in size in individuals. They commonly 

 contain 16 stigmata, but numbei's down to 12 were frequent and in one 

 case 10 only were present: only once more than 16 observed, and in 

 that case there were 23. Number of internal longitudinal bars on fold 

 varies from 6 to 9, generally 8 or 9. Hekdmax, p. 330. 



In considering the significance of these cases with reference to 

 the origin of Species it is to be remembered that the characters of 

 the branchial sac, the sizes of the transverse vessels, shape of 

 meshes and the number of stigmata they contain are held to be of 

 the first importance for the classification of Ascidians ; but Herd- 

 man finds that while they are highly characteristic in some species 

 they are not so in others 1 . 



II. Cyclostomi. 



*139. Myxine glutinosa. In this genus there are normally six 

 pairs of branchial pouches. I am indebted to Professor Weldon 

 for an account of a specimen dissected by him in which there were 

 seven pairs of these pouches. On the left side all the seven 

 pouches were distinct and separate, each having a separate open- 



1 The olfactory tubercle in Ascidians may have a different form and position in 

 different individuals of the same species, but the range of variation changes 

 according to the species. Molgula was found to be the most constant, Ascidia 

 virginea and A. plebeia the most variable forms. Hekdman, Proc. R. Phys. Soc. 

 Edin.,vi., p. '267, tigs.; also id., Proc. Lit. Phil. Soc. Liverpool, xxxvin. p. 313, 

 Pis. i. and ii. Variation respecting the atrial pore will be considered in connexion 

 with Bilateral Repetition. 



