ON THE ANOKSTRAL FOiiM OF THE CHOBDATA. 365 



Lang does upon those of Guiida; incipient coelomic sacs, 

 comparable to those of the larval stage of Amphioxus, 



A question very difficult to answer is this: How do these 

 alimentary diverticula eventually come to exchange their 

 function and significance to such an extent? If they were 

 originally acquired with a view to an enlargement of the diges- 

 tive surface, they must in the course of time, as they became 

 constricted off, have lost this function, and in its stead have 

 developed powerful layers of epithelial muscular tissue in their 

 walls, which then represent the successive myomeres, 

 and which finally supplant the original muscular 

 body-wall (Hautmuskelschlauch), itself never divided 

 into myomeres, and originally derived from the 

 epiblast. 



Traces of this epiblastic muscular sheath, primitively en- 

 veloping the myomeres, which secondarily spring from the 

 alimentary diverticula, appear to be found in certain Verte- 

 brates, externally to their general musculature. 



It remains for the present unsolved what were the leading 

 factors in this important transformation, the general outlines 

 of which we have here only touched upon. 



We have now to compare Nemertines and primitive Verte- 

 brates under another important head : foremost oesophageal 

 diverticula and their relation to respiratory functions and sen- 

 sorial (?) apparatus. Here, too, I do not wish to enter into a 

 thorough discussion of the subject ; an enumeration of the chief 

 points may suffice for the present. 



A special respiratory apparatus in the form of external 

 branchiae has never been met with in Nemertines. In a very 

 early stage of embryological development, however, two 

 lateral diverticula, situated in the very foremost 

 portion of the oesophagus in front of the mouth, 

 bud out from its wall (Biitschli, Barrois, and others), 

 and are in this stage directly comparable to similar 

 diverticula which arise in the same region in the 

 Balanoglossus larva, and there give rise to the first 

 pair of branchial slits (figs. 14 and 15), 



VOL. XXIII. NEW SKlv. B B 



