goldsohmidt's monograph of amphioxides. 587 



metrical in the larva of Ampliioxus, was originally sym- 

 metrical. 



The gill-slits of Amphioxides are arranged eumeta- 

 merically, that is to say, there is, in general, an exact 

 correspondence between branchiomerisni and myomerism in 

 the branchial region. An individual having twenty -nine 

 gill-slits, for example, will have thirty gill-arches, of which 

 the first arch corresponds with the first myotome, while the 

 thirtieth arch lies in the thirtieth segment. This applies to 

 two of the species, A. pelagic us and A. s ten urns. In A. 

 valdiviee repeated counting showed that there were more 

 gill-slits than myotomes in the branchial region, so that in a 

 specimen with twenty-seven gill-arches the last lay in the 

 twenty-third segment, or, more correctly, under the twenty- 

 third myotome. Such a specimen is interpreted as having 

 four supernumerary gill-arches ; the highest number observed 

 was seven. The supernumeraiy arches are considered to 

 indicate the occurrence of prosomital gill-slits in A. valdiviee. 

 This unexpected interpretation appears simpler in the writing 

 than in the illustration (fig. 2). Those who can appreciate 

 it will be able to make the best use of it. For my part, the 

 lack of coincidence between gill-slits and myotomes in A. 

 V aid i vise appears as an indication of the independence of 

 branchiomerisni and myomerism in the Acraniata, however 

 closely they may be correlated. 



In A. valdiviae the ventral halves of the anterioi myo- 

 meres are bent backwards at a very acute angle in correlation 

 with the great length of the mouth, 1 — 1'4 mm. (fig. 2). 

 This again strikes me as being a slight myomeric disturbance 

 independent of the gill-slits. 



Above the gill-arches and between every two gill-slits the 

 pars nutritoria of the pharynx is fused with the body- 

 wall, the lines of fusion coinciding with the dissepiments of 

 the myotomes. Between two lines of fusion the body-cavity 

 extends upwards as a pouch to the base of the notochord. 

 Seen from the side, the lines of fusion appear as semilunar 

 folds (fig. 2). This is the inter-segmental concrescence of the 



