456 E. RAY LANKESTBR AND ARTHUR WTLLEY. 



adult are represented by the region of longitudinally pleated 

 ventral wall between the two metapleura. 



The formation of the atrium as a narrow groove which 

 closes, sinks into, and expands within the body of Am phi- 

 ox us, is much more readily comparable to what is known 

 of the formation of the atrial chamber in the Ascidians than is 

 the Kowalevsky-Rolph scheme. In the Ascidian a pair of in- 

 pushings are formed, each with a circular orifice of invagina- 

 tion ; thev expand within the body, fuse with one another to 

 form one cavity, and one of the circular orifices disappears. 

 In Amphioxus we have a single in-pushing with a longitu- 

 dinal orifice of invagination, which closes as the invagination 

 forms, excepting at its hindermost border, and then expands 

 to a greatly increased volume. 



The comparison of the so-called epipleura of Amphioxus 

 with the opercula of fishes has only a remote morphological 

 basis, and probably no genetic relationship exists between 

 these two structures. On the other hand, it is very probable 

 that whilst the median fin-rays and fin including the ventral 

 fin with its double rays represent the median fins of fishes — the 

 metapleura represent morphologically the primitively con- 

 tinuous lateral fins. The duplication of the fin-rays in the 

 median ventral series of adult Amphioxus appears to be only 

 a complete carrying out of a tendency to bifid structure 

 which is found in the young dorsal fin-ray (see Lankester — 

 Amphioxus, ^ Quart. Journ. Micr. Sci.,' vol. xxix, PI. 

 XXXVI, B. fig. 11) ; and though in both dorsal and ventral 

 median fins the fin-ray lymph-space is single, yet the floor of 

 this space has a bilateral origin according to Hatschek. 



The figures which are given in PI. XXIX represent two stages 

 of the larvse of Amphioxus, an earlier with three gill-slits 

 and the rudiment of a fourth (figs. 1, 2, 3), and a later with 

 twelve gill-slits and the rudiments of two more (figs. 4, 5, 6). 



Though the older larva is considerably larger than the 

 younger, the two are, for the sake of comparison, represented 

 of the same size. 



