DEVELOPMENT OP ATRIAL CHAMBER OF AMPHIOXUS. 465 



Figs. 3 and 4. — Portions of sections through another larva of same age, 

 and prepared in same way as the last, taken just posterior to the region 

 represented in Fig. 2, to show the opening of the nephridium of Hatschek 

 into the mouth-cavity. Notice that the uephridiunj lies immediately below 

 the left dorsal artery. 



N.B. — In the larva there are not two dorsal arteries — right and left— in the 

 pharyngeal region, as there are in the adult ; but only one, and that on the 

 left side of the notochord. 



Fig. 5. — Section through first gill-slit of same larva, showing the club- 

 shaped gland opening into the mouth-cavity at its upper extremity. The 

 ventral vessel lies on the right wall of the intestine in the pharyngeal region. 

 No cavity yet in the right metapleur. 



Fig. 6. — Section through same larva as Fig. 1 (twelve gill-slits), through 

 the same region as preceding, to be compared with Fig. 5 (with eleven 

 gill-slits) where the mouth is half shut. In this case the mouth is wide open, 

 and the appearance of the section is considerably altered owing to the expan- 

 sion of the ventral portion of the ccelom. The right metapleur is more 

 advanced, but still has no cavity in this region. 



Fig. 7. — Section through the sixth gill-slit of the same larva. The double 

 appearance of the slit is due to a fold in the wall of the slit. The right meta- 

 pleur has a cavity here. The left metapleur has commenced as a thickening. 



Fig. 8.— Section through twelfth and last gill-slit of same larva. The 

 metapleural folds are nearly equal. There is a very small cavity in the right 

 and none in the left fold. 



Fig. 9. — Section through the post-pharyngeal region of a larva preserved 

 with osmic acid vapour, rather older than Fig. 8, but with no part of the 

 atrium floored in. The various divisions of the myocoel will be understood by 

 a reference to Hatschek's figures, reproduced in Professor Lankester's paper 

 in this Journal, vol. xxix, PI. XXXVIa, figs. 6 and 7. 



Fig. 10. — Section through the twelfth (last but one) slit of a larva of the 

 age of that represented in PI. XXIX, fig. 4. Preparation : concentrated 

 sublimate ; borax carmine. 



Fig. 11. — Section through tiie post-pharyngeal region of the same larva 

 (ef. Fig. 9), showing the fusion of the sub-atrial ridges. The character of 

 the latter as ridges on the inner faces of the metapleura is not so well seen 

 here as in other sections. 



Fig. 12. — Section through the same region of another larva of the same 

 age, showing the method of fusion of the sub-atrial ridges as described in the 

 letterpress. Preparation : osmic acid and picro-carmine. 



Fig. 13. — Section through the compound sense-organ (= prseoral pit) of a 

 larva in which all the gill-slits, except the first two, opened into a floored-in 



