AMPHIOXUS 15 



presence of the gill-slits. Since these slits are openings from 

 the gut to the outside (morphologically, ignoring the atrium) 

 they form connexions between the gut-wall and the body- wall, 

 and thereby necessarily obliterate the ccelom in places. The 

 ccelom, therefore, is restricted to the regions between the 

 slits, i.e. to the gill-bars. The coelom is perfectly normal 

 above and below the level of the gill-slits. Accordingly, there 

 are a pair of dorsal coelomic cavities, separated from one 

 another in the middle line by the dorsal mesentery ; and a 

 ventral coelomic cavity known from its position as the sub- 

 endostylar coelom. The latter is in open communication with 

 the dorsal ceeloms on each side by means of the coelomic 

 canals in the primary gill-bars. There are no coelomic canals 

 in the secondary or tongue bars, for they are later developments 

 which divide the original gill-slits into two. The relations of 

 the coelom are not difficult to understand when it is remembered 

 that relicts are left between the slits in the primary gill-bars. 



Into each dorsal coelomic cavity a conical outpushing of 

 the atrium projects, from the region just behind the gill-slits. 

 The apex of the cone points forwards, and so lies dorsal to 

 the hindmost gill-slits. These structures are the so-called 

 " brown funnels," of doubtful significance (see p. 12). 



Excretory System. — The excretory organs of Amphioxus 

 are remarkable in that they are nephridia. In all other 

 chordates the excretory organs are coelomoducts or meso- 

 dermal kidneys (see p. 31). The nephridia lie over the gill- 

 slits, project into the dorsal coelomic cavities and extend a 

 short way down the coelomic canals in the primary bars in the 



Fig. 10. — Amphioxus : transverse sections through the body in the regions 

 of A, Kolliker's pit ; B, Hatschek's pit ; C, the anterior region of the 

 pharynx ; D, the posterior region of the pharynx ; E, between pharynx 

 and atriopore ; F, the atriopore ; G, between atriopore and anus ; 

 H, the anus. 



a, anus ; op, atriopore ; at, atrium ; be, buccal cirrhi ; bf, brown funnel ; 

 c, ccelom ; da, dorsal aorta ; dc, dorsal coelomic canal (in the region of the 

 pharynx) ; df, dorsal fin ; dn, dorsal nerve-root ; e, eye-spot ; en, endostyle ; 

 ep, epipleur ; fr, fin-ray box ; g, gonad ; gs, gill-slit ; hg, hyperpharyngeal 

 ciliated groove ; Hp, Hatschek's pit ; i, intestine ; Kp, Kolliker's pit ; /, 

 liver ; Ida, lateral dorsal aorta ; m, myotome ; mc, myocoel ; mp, meta- 

 pleural fold ; n, notochord ; nc, nerve-cord ; oh, oral hood ; p, pharynx ; 

 pa, extension of the atrium behind the atriopore ; sc, subendostylar coelomic 

 canal ; vf, ventral fin ; vn, ventral nerve-root. 



