THE STRUCTURE OP XENIA HICKSONI. 301 



more clearly seen. Note also tliat the polyps are smaller and closer together 

 near the edge than nearer the middle of the summit. The stem immediately 

 to the right of this also shows that the young and small polyps are on tiie 

 edge, while the large polyps are near the middle of the summit. The colour 

 of the drawing is, as nearly as possible, the colour of the specimen in spirit. 

 X 3. 



PLATE 24. 



EiG. 2.— View of the polyp XXI in the table, p. 284. This is one of the 

 largest polyps of the colony. The tentacles show from the outer aspect a 

 single row of pinnules on each side, but from the inner aspect three rows on 

 each side. Note that at the tip of the tentacles the pinnules are not arranged 

 in pairs. X 12. 



EiG. 3.— Views of the tentacle of a large polyp. A and B are views of 

 ihe tip, C and B of the base of a tentacle. A and C show the outer or aboral 

 side, B and D the inner or oral side. Tiie narrow area free from pinnules, 

 which extends along the nuddle line of the oral face of the tentacle, is well 

 seen in D. At the tip of the tentacle, B, the pinnules are in two rows only 

 on each side of the middle line. X 24. 



YiG. 4. — Lateral view of the youngest polyp (-32 mm. long) in the colony 

 (I in table). X 40. 



Tig. 4a. — Oral view of the same polyp. The tentacles are eight rounded 

 lobes. The depression {Mo.) in the centre of the oral disc indicates the 

 position of the future mouth, and the darker area below it is the ingrowing 

 plug of ectoderm which forms the stomodaium. X 40. 



Fig. 5. — An older polyp '95 mm. long (V in table). The tentacles are 

 trilobed at the end, i. e. the first pinnules are being formed. X 24. 



Fig. 6.— a polyp 1-42 mm. long (VIII in table), x 24. 



Fig. 6a. — View of the oral face of a tentacle of the polyp shown in fig. 6. 

 Two rows of ninnules on each side of the middle line are now present, x 24. 



Fig. 7.— a polyp 2-27 mm. long (XII in table). X 24. 



Fig. 7a. — View of the oral face of a tentacle of the polyp shown in fig. 7. 

 In the middle part of the tentacle, three rows of pinnules on each side of the 

 middle line are now distinguishable. X 24. 



PLATE 25. 



Fig. 8. — A thick longitudinal section through the upper part of one of the 

 stems. The section passes along the dorso-veutral axis of a young polyp (VI 

 in table, p. 284) growing out just under the arched summit of the stem. On 

 the dorsal or upper side of the polyp one of the dorsal mesenteries {B. M.) is 

 cut through obliquely for a short distance. The stomodseum {St.), siphonoglyph 

 {Si.), the course of the dorsal mesenterial filaments {D. M. F.), the thin edge 



