42 ANATOMY OF AMPHIOXUS. 



pharynx are special developments of the basement membrane, 

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

 from one another. (Cf. Fig. 15.) More recently, BENHAM 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- 

 KESTER supposed it to be a diverticulum of the coelom. SPEXGEL 

 and BOVERI interpreted it as a blood-vessel ; and, finally, BENHAM 

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

 vessel contained in a ccelomic space. It should be added that 

 these finer details are extremely difficult to determine. 



7. (p. 21.) Lateral Line. Since the lateral line constitutes 

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

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

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

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

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

 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 the Vertebrates. The modifications which it 

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

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

 ferent parts. Thus, while the metapleural folds are very probably 

 the homologues of the primitive continuous lateral fin-folds, yet 

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

 the primordial condition of these folds. Certain peculiar features 

 in connexion with the origin and innervation of the metapleural 

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

 origin of the lateral line sense-organs which may perhaps have 

 some value as a working hypothesis. 



In those primitive fishes which possessed the continuous lateral 

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

 a locomotor function, but they must have served primarily as 

 balancers. Without going into the difficult question as to how 



