172 MKRISTIC VARIATION. [PART I. 



*1.S">. Ascidia plebeia 'Alder): branchial sac has very characteristic 

 appfaraii. and is very constant in the size of meshes, papillae &c. 

 < >nc point is liable to variation: as a rule the transverse vessels are of 

 tin- .-aim- calibre. but in several specimens every fourth vessel is much 

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



Stiff niata and Meshes. 



i:lt!. Ciona intestinalis : meshes vary but according to no apparent 

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

 1" tin- utmost seen. HERDMAX, p. 332. 



1 '". Ascidia aspersa. In typical specimens, transverse vessels all same 

 size, tin- meshes being square and undivided, but individuals occur in 

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

 tnuis\ .-r>H vessels into pairs of oblong areas. HERDMAX, p. 332. 



I:;N Styela grossularia. The genus Styeta is characterized by the 

 pn-M-iice of branchial folds, normally four on each side, but in this 

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

 ',< !> -ide ami reduced to a single slight inward bulging on the right 

 -ide. Itc.irin^ internal longitudinal bars. This fold is separated from 

 tin- dor-al lamina by a broad space without internal longitudinal bars. 

 \ -imilar wide space is present on the left side of the dorsal lamina, 

 ami two others .>n the vertebral edge of the sac, one on each side of the 



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



ontain hi stigmata, but numbers down to 12 were frequent and in one 

 case 1 () only were present: only once more than 16 observed, and in 

 tliar cose t/ien were .''. Number of internal longitudinal bars on fold 

 varies from >'> to !t, gem-rally 8 or 9. HERDMAX, p. 330. 



In considering tin- significance of these cases with reference to 

 :ln- origin of Species it is to be remembered that the characters of 

 In- branchial sac, tin- sizes of the transverse vessels, shape of 

 meslie-. and tli.- number of M iumata they contain are held to be of 

 In- first importance for the dassitjeation of Ascidians ; but HERD- 

 MAN finds that while tin-van- highly characteristic in some species 

 rhrv are not 80 in others'. 



m 



I I. t !Y< l.oST't.MI. 



*!:}!>. Myxine glutinosa. In this i^-nus then- are normally six 



pairs .,)' branchial pouches. I am indebted to Professor \Veldon 



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



n pairs <.f these pouches. On the left side all the seven 



pouches W ere di-tinet and sepai-ate, each having a separate open- 



Tin- ..Ifii.-t.iry tnli.Ti-lr in A^'idiaiis may ha\v a .lilY.-rcnt form and position in 

 t individuals of tin- -am.' spcc-irs, l.nt tli.' range of variation changes 

 iiK t.. thr species. .!/.. /.'/(//</ was t'.nnnl t> l>c tli.- most constant, Ascidia 

 rir<nni ami .1. /./,/.,//( tin- m.i-t varialilr t'..rms. HCKLMAN, I' roc. J\. PI/ 1/*. Sue. 

 /../i/i., vi., p. -_'C,7, figs.; also //., I'm,-. 1. it. Phil. S,,c. Lir,T]>in>l, xxxvin. 'p. 313, 

 I'ls. i. ami n. Vanati.'ii i.-^pirtinj.' the atrial p..n- will be con-i.li'i-.-.l in connexion 

 -. ith I'.ilat.-ial Repetition. 



