42 .t.V.irOA/V OF AM/VZ/OXUS. 



pharynx are special devoloiMnents of the basement membrane, 

 wliich separates the two opposed epithelial layers of each gill-bar 

 from one another. (Cf. Fig. 15.) More recently, Benh.\i\i has 

 described nuclei in the latter membrane, thus showing it to be a 

 sheet of connective tissue. In this case the substance of the 

 skeletal rods should be regarded as a variety of connective tissue. 



A further difference of opinion prevails as to the nature of the 

 space which traverses the skeletal rod of the tongue-bar. Lan- 

 KKSi'KR supposed it to be a diverticulum of the ccclom. Spengel 

 and Bovi'-.Ri interpreted it as a blood-vessel ; and, finally, ISenham 

 thinks that it is bodi, inasmuch as he conceives there to be a blood- 

 vessel contained in a ctelomic space. It should be addetl that 

 these finer details are extremely tlifficult to determine. 



7. (p. 21.) Lalcral fjiif. — Since the lateral line constitutes 

 one of the most characteristic and constant features in the organi- 

 sation of fishes, its absence in Amphioxiis has always been one of 

 the most serious difliculties in the way of a conception of this 

 animal as, in any sense, an ancestral form. It need hardlv be 

 pointed out that from whatever point of view we regard Aniphi- 

 oxus, it must necessarily have become specialised and modified 

 along its own particular line of evolution, and cannot, as it stands, 

 be taken as a direct ancestral form, but rather as a more or less 

 close relative of, or an exceedingly ancient offshoot from, the 

 actual ancestor of tlie Vertebrates. The modifications which it 

 has undergone will, as in every other case, have resulled in more 

 or less extensive changes both in the function and structme of dif- 

 ferent parts. I'hus, while the metapleural folds are \er\- probably 

 the homologues of the primitive continuous lateral lin-lulds, \et 

 in their actual form and finiction they may or may not represent 

 the primordial condition of these folds. Certain |>e(uliar features 

 in connexion with the origin and innerv.ition of the mctaiileural 

 folds of Ampliioxus have led me to form a conception as to the 

 origin of the lateral line sense-organs which may perhaiis ha\'e 

 some value as a working hypothesis. 



In those jirimitive fishes which possessed the continuous lateral 

 fin-folds, it is very clear that the latter could not have perfiirnied 

 a locomotor fimction, but they must have servetl primaril\ as 

 balanccis. Without gning into the difficult (|uestion as to how 



