THE STRUCTURE OP XENIA HICKSONI. 233 



Fig. 31. — Section of a sperm sac a little older than the largest one shown 

 in the preceding figure. The central cavity containing a coaguluni has now 

 appeared. x 150. 



Fig. 32. — Section of an older sperm sac. x 150. 



Fig. 33. — Section of a mesentery, on the end of which is a sperm sac in 

 which the central cavity, having reached its greatest size, is now being 

 encroached upon by the heads of ripening spermatozoa. Note that the 

 spermatozoa are surrounded by a thin film of mesogloea (represented in the 

 distal part of the sac by the line inside the endoderm), and by an endodermic 

 follicle, some of the cells of which contain from two to five nuclei. x 150. 



Fig. 34. — Ripe spermatozoa. 



Fig. 34a. — From a section of the stomodasum, through the dorsal portion 

 of which spermatozoa are escaping. Only the head and point of attachment 

 of the tail are seen. x 800. 



Fig. 34b. — An entire ripe spermatozoon from a teased preparation of a large 

 sperm sac. X 800. 



Fig. 35. — Cells from thin sections (4 p thick) of large sperm sacs, to show 

 the two to five nuclei -which may be present in each. x800. 



Fig. 36. — Transverse section of a ventral mesentery of a young polyp 95 

 mm. long (V. in table, p. 215 ; see also PL 21, fig. 5). The section is taken 

 about - 5 mm. below the lower end of the stomodasum. Note the already 

 differentiated primitive genital cells migrating from the endoderm into the 

 mesogloea, and the short finger- shaped flagella of three of the endoderm cells, 

 x500. 



Fig. 37. — View of a colony of Heteroxenia Elizabethce. The colony was split 

 in two longitudinally and the proximal half drawn, hence only half the summit 

 of the stem is seen. Note the autozooids with pinnate tentacles, and the very 

 numerous short siphonozooids with short rounded tentacles, and also the young 

 polyps growing near the edge of the summit. X 3. 



Fig. 38. — View of the youngest autozooid on the colony, "64 mm. long (A. 1 

 in table, p. 218). The specimen has been flattened out by pressure against the 

 side of the bottle which contained the colony. Note the long finger-shaped 

 tentacles ; the right proximal one is slightly indented near the tip, but the 

 others have simple rounded ends. The stomodasum is indicated. x 40. 



Fig. 39. — The largest siphonozooid on the colony (S 7, Fig. 38, and table, 

 p. 218). The short simple tentacles, the stomodseum, and the two dorsal 

 mesenteries with their filaments are shown. x 15. 



