2.'>S ' PERCY SLADEN TilUST EXPEDITION". 



restricted and takes on the character of a true epibrancliial i^roove, liaed by tall 

 columnar cells. 



Tne failure to find ^peripharyngeal bands or excretory canals was no doubt due to the 

 bad preservation of the pharyni^eal wall. 



The region behind the gill-slits need not detain us. Tlie atrium here encroaches on 

 the epibranchial ccelom considerably more than it does in the gill-region. There is no 

 sign of any post-atrioporal extension of the atrium. 



The metapleural folds are for the great part of their extent — from the level of 

 the 3rd gill-slits to the atriopore — symmetrical. The anterior, asymmetrical portion 

 of the right fold somewhat recalls that of A. pelaglcus in the manner in wliich it 

 merges into the right clieek, and in the course of its band of clear cells, which can 

 still be traced, although it shows signs of breaking up, and merges anteriorly into 

 an extensive tract of thickened epidermis ; the left metapleure, likewise, still possesses 

 its band, and for some distance behind the point where the two folds become symme- 

 ti-ical their two bands are seen lying side by side on the shelf of tissue which floors 

 the atrium. 



The left metapleure dies out a little behind the opening of the buccal cavity, just 

 internal to the ridge (seen in PI. 15. fig. 16) which forms the prolongation of its left edge. 

 Its anterior portion thus lies much more to the left tlian in Amphioxides : it also shows 

 a very marked difference in point of size. 



The shape of the metapleures, and the pronounced constriction by which they are 

 marked off from the body, can be seen from fig. 18. The well-developed pterygocceles 

 are roofed dorso-medially by the pterygial muscles, covering wliich no epithelium can 

 be seen except a band of peculiar large cells on either side. Behind the gill-region the 

 pterygocoele comes to occupy the whole interior of the metapleure, the gelatinous layer 

 of the cutis disappearing completely. 



Behind the atriopore, the left metapleure soon dies away, while the right becomes 

 continuous with the ventral fin, a chai-acter shared by the genera Heteropleuron and 

 Asymwetron, but not found in Branchlostoma. Andrews describes the posterior 

 portions of both metapleures in the second named genus as having the structui-e of 

 tins: this is not the case here, since botli pterygocceles are stiU visible behind the 

 atriopore. They appear to end by opening into the splanclmopleure, but, as is so 

 frequently the case, the openings cannot be determined with absolute certainty. 



Vascular System. — The aorta is single, as in the larva, in the posterior gill-region: 

 further forward it divides into two branches, of which the left can be traced as far as 

 llatschek's pit — it overlies Hatschek's nephridium anteriorly. The right 'branch tapers 

 away behind the velum. In front of the latter traces of the so-called "right aortic 

 arch " can be seen as a plexus of vessels lying below the notochord, apparently com- 

 municating with the left aorta anteriorly. 



The dorsal and ventral fins are similar in structure to those of Amphioxides: 

 towards the hinder end of the tail, at any rate, the fin-strands appear to be quite sohd ; 

 more anteriorly they may become hollowed, but do not open into the fin-ray boxes. 

 As in Amphioxides and Asymmetron, the ventral fin is displaced to the right by the 



