464 E. RAY LANKESTER AND ARTHUR WILLEY. 



pleur. s. a. r. Sub-atrial ridges or floor, s. o. Seuse-orgaii (part of prajoral 

 pit), som. Somatopleur. sp. Splanclinocoel. sp. p. Splauchuopleur. v. a. 

 Ventral vessel, w. o. Ciliated organ (of prseoral pit). 



General Remarks. 



The intestinal epithelium is ciliated throughout. The epithelium bordering 

 the gill-slits is much modified, being divided up into innumerable small cells, 

 the cell-divisions between which cannot be seen under ordinary circumstances. 



The nerve-cord consists of a nucleated portion surrounding the central 

 canal and a peripheral fibrous portion. 



Nuclei are to be seen in the notochord, and in the superior and inferior 

 canals of the notochord. 



There are nuclei in the muscle-plates, but, as Hatschek points out, there is 

 no epithelium on the outer wall of the muscle-plates. The nuclei on the 

 inner wall are sufficiently scanty. 



The sense-organ and ciliated organ of the prseoral pit are derived together 

 from the left anterior diverticulum of the archenteron of the embryo, while 

 the right diverticulum becomes simply the space occupying the anterior end 

 of the body. It is included in Fig. 1, but not in Fig. 13. 



A reference to the drawings of the whole animal in PI. XXIX will show 

 approximately through what regions the sections have been taken. 



Figs. A, B, C. — Three diagrams of larvte, seen from ventral aspect, to 

 illustrate the origin and relation of the metapleural ridges to one another, 

 and the gradual closure of the atrium from behind forwards. 

 Fig. A. No atrium. 

 Fig. B. Atrium behind pharynx. 



Fig. C. First two gill-slits open to exterior, all the rest now open into 

 the atrium. 



Fig. 1. — Transverse section through the region of the ciliated organ and 

 sense-organ of the prseoral pit, just in front of the opening of the latter into 

 the former. The anterior commencement of the splauchnoccel, and the pos- 

 terior portion of the right embryonic diverticulum are shown. The epithelium 

 of the prseoral pit is of hypoblastic origin (Hatschek). This larva had twelve 

 gill-slits, and no closed atrium. Preparation : osmic acid, borax car., fol- 

 lowed by Meyer's carmine. 



Fig. 2. — Transverse section through the commencement of the mouth- 

 opening, showing the external aperture of the club-shaped or tubular gland. 

 It also passes through the tract of modified epithelium. The very thin piece 

 of epithelium, two thirds of the way up, is the cause of the clear space or line 

 which gives a double appearance to the tract. The thickening of the right 

 metapleur is tending to the right side. This larva had eleven slits. Prepara- 

 tion : sublimate and acetic ; lia;matoxylin. 



