228 PERCY SLADEN TRUST EXPEDITION. 



fio-s. 6-7). At the end of the latter, it appears, in A. pelagicus at any rate, to become 

 directly continuous with the aorta, where both vessels terminate, immediately behind 

 Hatschek's pit. 



The anus lies almost exactly in the morphological middle line, and actually opens 

 towards the right, as Goldschmidt's fig. 66 shows. The ventral fin is thus displaced 

 fi'om its median position (or retains its primitive lateral position, according to another 

 view of its nature), exactly as Andrews has noted it to be displaced in Asymmetron — 

 an important point of correspondence. In Branchiosfoma, though formed close to the 

 middle line, the anus soon moves up to the left ; according to Hatschek's account (i88i ) 

 it is even formed slightly to the left. 



The Metapleural Folds. 



Goldschmidt has fully described the characteristic differences between A. valdivitn 

 and A., pelagicus in the matter of their metapleura. In the former they are thinner and 

 more distinctly marked off from the body, and contain throughout a well-developed 

 cavity ; the right predominates considerably over the left : in the latter they are stout 

 and consist largely of gelatinous tissue, the pterygocoeles being greatly reduced, and 

 they are not so asymmetrical. In both species the right fold ends near the level of the 

 anterior end of the mouth, the left near that of its posterior corner : the portion of the 

 right fold corresponding to the elongated mouth in A. valdivicB is particularly well 

 developed, overhanging the anterior gUl-slits, Avhich in this species lie well up in the 

 topographical right wall of the pharynx. 



So far I can corroborate Goldschmidt, but in further details I find several differences. 



Eirst — a point to which he makes no reference — both metapleui-es are marked 

 throughout the greater part of their length by a remarkable band of epidermal cells, 

 which can hardly be other than a sense-organ, though I have not been able to trace any 

 nerves into connection with it. The structure of this band, as seen in transverse section, 

 is roughly shown in PL 15. fig. 12. It consists of very large clear cells, with round 

 nuclei near their inner ends ; between their outer ends lie other cells with elongated 

 nuclei * ; the adjoining cells on either side have elongated and in some places appear to 

 have growTi round both externally and internally to the clear cells, so as to form a 

 canal, in which these lie : the elongation of the lateral cells affects their nuclei to a 

 remarkable extent ; they have almost the appearance of being cornified. 



Similar bands of cells occur on tiie metapleures of the Amphloxus larva, but do not 

 seem to have met with much attention. Lankester and Willey (1890) figure them in 

 young larvae ; according to these authors they migrate inwards before metamorphosis. 

 Macbride(i898) describes the right band as an "epithelial thickening" forming" the first 

 trace of the future fold on the right side." Subsequently (1900) he takes the view that 

 the definitive pterygocoeles are formed by the hollowing out of groups of ejnthelial 

 cells which have wandered into the interior of the folds, and suspects these to be the 

 " large clear cells " seen in earlier stages. It is evident, from the conditions found in 

 Amphioxides, that this view cannot be correct. 



* I am iudebted to M. Legros for pointing out the existence of this second type of cell. 



