466 E. RAY LANKESTER AND ARTHUR WTLLEY. 



atrium. It shows the sense-orgau {s. o.) opening into the ciliated organ (w. o.), 

 and the latter opening widely to the exterior. It also shows the independent 

 origin of tlie oral hood (o. k.). Preparation : concentrated sublimate ; borax 

 carmine. 



Fig. 14. — Section through the anterior opening of the atrium, in the same 

 larva. It shows very well the position of the atrium anteriorly on the right 

 side, also the sub-atrial ridges. Note the relatively huge size of the right 

 metapleur, and the almost entire absence of any indication of the left meta- 

 pleur; thus showing that the sub-atrial ridges are distinct structures from, 

 and only secondarily dependent on, the metapleural fplds ; and that the latter 

 serve a function (probably vascular) other than that of merely contributing 

 to the formation of the atrium. This section is between the second and third 

 gill-slits. The second slit opens to the exterior, the third opens into the 

 atrium. The large cells at the bottom of the right metapleur are still in the 

 epidermis ; whereas, in fig. 7, PI. XXX, they have migrated inwards. 



Fig. 14 a. — Section through the same larva as the preceding, two or three 

 sections farther back, showing a gill-slit (the third) opening into the laterally 

 placed atrium. 



Fig. 15. — Section between the tenth and eleventh slits of the same larva, 

 showing an older condition of the atrium than that represented in Figs. 11 

 and ] 2, with " renal " cells on the dorsal wall. Note also the large size of the 

 metapleura. 



Fig. 16. — Section through the post-pharyngeal region of the same larva, 

 showing a still more advanced condition of the atrium. The gelatinous sub- 

 cutaneous tissue has disappeared from the dorsal wall of the atrium, leaving 

 a thin double membrane, consisting of ccelomic and atrial epithelium 

 (= somatopleur). 



Fig. 17. — Section through the atriopore of same larva. 



Fig. 18. — Section through the last gill-slit but two of a larva with the 

 atrium floored in over three slits, showing expansion of atrium and temporary 

 obliteration of metapleural spaces. Preparation : osmic acid and picro- 

 carmine. 



Figs. 19 and 20. — Sections through a larva in which the atrium had closed 

 over two slits (the fourteenth and fifteenth), showing a narrow condition of 

 the atrium in front (Fig. 19), followed by a more expanded condition behind 

 (Fig. 20). 



