G8 DR J. H. ASHWOKTH 



Fig. 4. Outer surface of the ventral valve of the largest larva, 16 mm. long (larva d, group v, p. 52) ; 

 to show the protegulum and the principal growth-lines on the secondary shell. See p. 53. ( X 39.) 



Fig. 5. Larva with 1 1 pairs of cirri, ventral aspect ; drawn from a preserved specimen. The anterior 

 and posterior occlusors and the lateralis muscle are shown, but the other muscles have been omitted. The 

 cadomoducts were present, but have not been shown ; they are more clearly seen in later stages (see fig. 10). 

 The cirri are contracted, and the mantle, which in life extends to the margin of the shell-valves, has also 

 been withdrawn. The right and left lobes of the ventral " liver " are seen to unite in front and to open 

 into the stomach by a common portion (see also fig. 10). Between the posterior margin of the lophophore 

 and the anterior edge of the ventral nerve-ganglion in the body-wall there is a deep depression shown in 

 light tone in the figure. Note the first indication of the peduncle. See p. 49. ( X 90.) 



Fig. 6. Posterior end of a larva with 14 pairs of cirri (larva a, group v, p. 52), ventral aspect. Note 

 the peduncle arising from the ventral mantle-fold, turning to the animal's right, then dorsally and left, and 

 terminating in a knob lying between the dorsal and ventral mantle-folds. The narrow canal connecting the 

 peduncular cavity with the ccelom is indicated. In the mantle on each side are chsetse and a series of 

 gland-cells. Two of the chsetae — those first formed — are rather larger than the rest. The dark central 

 area indicates the extent of the body proper. ( X 90.) 



Fig. 7. The distal half of one of the large chsetse of a larva with 15 pairs of cirri (larva d, group v, 

 p. 52). (X500.) 



Fig. 8. Horizontal section of the dorsal region of a larva (_/, group iv, p. 50) with 13 pairs of cirri. In 

 the posterior part the section contains the dorsal body-wall (which is intact) in the anterior region of which 

 are the two statocysts. Further forward the dorsal body-wall has been cut away. The two antero-dorsal 

 " liver "-lobes are coming together in the middle line preparatory to opening into the stomach. The doi'sal 

 blood-vessel is seen through the dorsal body-wall, and a small portion of the vessel is cut again further 

 forward in the dorsal mesentery. ( X 100.) 



Fig. 9. Horizontal section, 20/x thick, of the same larva, taken nearly midway between dorsal and 

 ventral surfaces. The section passes through the oesophagus and stomach and the postero-dorsal " liver "- 

 lobes. The cirri are drawn only in outline ; note in the lophophore on each side a portion of the arm-sinus 

 and of the band of supporting substance lateral to it. Only the basal half of the tentacle is present in 

 the section. ( X 100.) 



Fig. 10. Horizontal section, 50/x thick, of the body proper of the same larva, taken in the ventral region, 

 showing the posterior portion of the stomach (cut tangentially), the intestine, the ccelomoducts (nephridia), 

 and the ventral " liver." The median anterior portion of the " liver " leads into the stomach, which, in that 

 area, lies immediately dorsal to the plane of the section. Note also the ventral nerve-ganglion in the 

 anterior wall of the body. ( X 100.) 



Plate V. 



Pclayodiscus (Discinisca) atlanticus (King). 



Fig. 11. Larva, ventral aspect. Between the posterior margin of the lophophore and the anterior edge 

 of the body-wall there is a deep depression (cf. fig. 17). The dark tone on the mid-gut indicates the area 

 occupied by the flagellated cells, i.e. the area which will form the chief part of the stomach. Note the 

 intestine, ccelomoducts (nephridia), and peduncle. The five principal cluetse are shown on both sides ; but 

 on the right of the figure only are represented the three jointed chaeta? which are situated in the ventral 

 mantle-fold, and the first two and the last of the series of about thirty slender chsetre present in the dorsal 

 mantle-fold. The mantle, which extends to the margins of the shells, is extremely thin over the greater 

 part of its extent, but its thickened zone (ef. fig. 17) is represented by the deeper tone. ( X 180.) 



Fig. 1 2. Dorsal aspect of the same larva, showing only the body proper. Note especially the statocysts, 

 and the " eyes." The thickening of the body-wall in front is due to the presence on each side in that 

 region of the lateral nerve-ganglion. ( X 180.) 



Fig. 13. Distal end of the first left chseta. See p. 60. ( X 800.) 



Fig. 14. The third principal chuvta. The processes are on its anterior margin. See p. 61. ( X 800.) 



Fig. 15. The distal end of the fourth principal cha-ta ; note the absence of spines on its median margin. 

 Seep. 61. (X800.) 



