58 Notes from the Qatty Marine Laboratory. 



Fig. 22. Transverse section of the region in front of fig. 21, in which the 

 opercular stalk is separating and the slits {hf.) for the forma- 

 tion of the branchial filaments making rapid progress on the 

 other side. On the ventral surface (left in the figure) the 

 thoracic collar is free. Similarly magnified. 



Fig. 23. Transverse section after the separation of the opercular stalk 

 and when slits are appearing on the left or opercular side 

 (upper in the figure). The great expanse of the vestibule, d., 

 is noteworthy; n., branchial nerve, the others lie toward the 

 inner ends of the slits. Only the inner brauchial nerve, n., is 

 indicated in this figure. 



Fig. 24. Transverse section of the tip of a branchial filament of the fore- 

 going. The blood-vessel occupies the centre. It is richly 

 ciliated in life. X oc. 2, obj. D. 



Fig. 25. Longitudinal section of a filament of Pomatocerus triqueter, L., 

 with portions of pinnules. X oc. 2, obj. D. 



Plate V. 



Fig. 26. Transverse section of the anterior region of Pomatocerus tri- 

 queter, L., with the thoracic glands, tg., in full development, 

 that on the left showing the origin of the duct which joins 

 that of the opposite side at the median outlet (fgo. in PI. III. 

 fig. 15 for Bispira). cm., cellular appendix of the thoracic gland; 

 d., oesophagus with its chloragogenous coat; dm., dorsal longi- 

 tudinal muscles; hyp., modified hypoderm ; m}, special anterior 

 median muscular layer on the dorsum ; m. 2 , special ventral 

 layer of muscle ; nc, nerve-cords. Above the gullet is the 

 dorsal blood-vessel in the mediau mesentery, and a space 

 occurs above it between folds of mesentery, but soon disappears. 

 X about 35 diam. 



Fig. 27. Transverse section of the body-wall toward the posterior region. 

 The dorsal muscles, dm., are of great size, with scarcely a 

 trace of separation in the mid-dorsal line ; vm., ventral 

 muscles; vv., ventral vessel with chloragogenous cells ex- 

 ternally. The outline of the gut is T-shaped. X about 35 

 diam. 



Fig. 28. Transverse section of an anterior foot with the hook-pad about 

 the level of the diminishing thoracic gland, tg. ; ab., incipient 

 muscular fibres of the process opposite the external papilla, p. 

 In this section none of the peculiar clavate nucleated cells are 

 visible, x oc. 2, obj. A. 



Fig. 29. Section behind the foregoing cutting the superficial part of the 

 hook-pad, and showing the greatly developed hypodermic 

 cells with the nuclei situated externally, and forming an elastic 

 cushion in connection with the dense row of minute hooks. 

 tg., thoracic gland. X oc. 2, obj. A. 



Fig. 30. Transverse section of the opercular stalk in another example, in 

 which the central area is chordoid or areolar in aspect. The 

 nerves are not entered. X oc. 4, obj. A. 



Fig. 31. Transverse section of the alimentary canal, showing lateral folds 

 of the mucous membrane, almost valvular in appearance. 

 X 350 diam. 



