LATER LARVAL DEVELOPMENT OF AMPHIOXUS. 217 



not one after the other, as is usual with metamerised organs, 

 but simultaneously. The successive or metaraeric mode of 

 formation, in fact, obtains theoretically with the secondary 

 slits of the larva of Amphioxus, only in this case it is latent, 

 with the natural result that the slits appear simultaneously. 

 The usually but not invariably late appearance of the first 

 secondary slit is due to a further slight retardation, and is 

 probably of no great significance. 



The morphological mid-ventral line of the larva is indicated 

 by the blood-vessel which lies dorsal to the primary gill-slits 

 (fig. l,&c.). 



While the lateral mouth is serviceable to the pelagic larva, 

 it would plainly be awkward to the animal during its career as 

 a dweller in the sand — hence the necessity for the transforma- 

 tion which has been described above. 



I have now given a possible explanation of the asymmetry 

 of the larva of Amphioxus, namely, that it can be traced ulti- 

 mately to the adaptive forward extension of the notochord. It 

 is thus a purely ontogenetic phenomenon, and is not an 

 ancestral character. It is only a temporarily superinduced 

 condition, and is destined to give place after a brief existence to 

 the inherent but latent bilateral symmetry of the organism. 



Amphioxus and the Ascidians. 



To pave the way for considering the significance of the endo- 

 style and club-shaped gland of the larva of Amphioxus, we 

 must institute a comparison between the embryo and larva of 

 an Ascidian and of Amphioxus. The remarkable researches 

 of Professors Ed. van Beneden and Charles Julin on the 

 morphology of the Tunicates (6) have rendered it possible to 

 do this the more effectually. 



The peculiarities of an Ascidian embryo as described by the 

 above-named authors are as follows (see Woodcut, fig. 1): 



The enteric cavity at first of two portions, viz. a prse- 

 chordal and a subchordal portion ; the greater part of the lumen 

 of the latter portion disappears at a very early period — it has 

 been identified with the post-anal gut of higher Vertebrates 



