EXPLANATION OF THE PLATES. 



{• 



Fig. 24. A young liydromodusarium. 



Fig. 2.5. Birils-eye view, sliowiug the interior of tlic 



broadly expanded mouth. 

 Fig. 26. Tlie buccal tentacles, so laid together as to 



resemble a solid ribbed mass. 

 Fig. 27. The medusifeious branches turned toward the 



mouth, so as to show their basal connection with the 



disk of the hydra. 

 Fig. 28. The buccal tentacles retracted, and the mouth 



wide open. 

 Fig. 20. A young hydroid, partially contracted. 

 Fig. 30, A young hydroid, contracted as in fig. 22. 



PLATE XXIII. 



P.^RYPU.V CROCE.i Ag. 

 [Figs. 1, lb, and Ic, drawn by A. Sonrel; the others by IT. J. Clark.] 



Figures 2*, 3, 4S 5, 7, 8, 9, 10, 11, 12, 12c, 13, 14, 15, 

 21, 22, and 23, are magnified 100 diameters; figs. 1^, 

 9S and 14", 200 diameters; figs. 3a, 4, .5a, 7a, 18, 18^, 

 and 19, 300 diameters; figs. C, 12*, 12b, 19a, 21*, 22", 

 23a, and 24, 400 diameters; figs. 9b, 15a, igb^ 17^ ija^ 

 25, 2G, 26a, and 2Cb, 500 diameters. 



Fig. 1. A group of immature hydroids. a a' a" a' 

 branches and stolons ; b c d e f g the heads in differ- 

 ent stages of growth. 



Fig. la. A full-grown liydromodusarium. a b the stem; 

 a' the stolon; c the medusa; bunches; J base of the 

 head; e proboscidal tentacles. 



Fig. lb. The head and top of the stem of a hydro- 

 medusarium, fi-om fig. 1. a b c c^ the medusa;; d the 

 stem; rf' top of rf; e e' branchlets of the medusiferous 

 branch; ;) the proboscis; I buccal tentacles; (' coro- 

 nal tentacles. 25 diameters. 



Fig. 1». The proboscis of fig. lb opened longitudinally. 

 m mouth ; p the walls ; p' pfl internal folds ; t buccal 

 tentacles; (' decurrent base of buccal tentacles; C- 

 eentripetal bases of buccal tentacles. 



Fig. IJ. The chitinous sheath. 3 diameters. 



Fig. le. The end of a buccal tentacle. a outer, and 

 b inner wall ; c dense accumulation of lasso-cells. 200 

 diameters. 



Fig. 2. A bunch of male medusa;, ab\ c the branch 

 from which they arise. 25 diameters. 



Fig. 2a. A male medusa, a pedicel; b disk; c sper- 

 matic mass; d proboscis. 



Figs. 3 to 2Cb represent the development of the medusa 

 and its young. 



Figs. 3 to 7 are lettered ahke. a outer wall of the 



medusa : a' outer wall of the branch or pedicel ; h 

 inner wall of the medusa ; l!" inner wall of the branch 

 or pedicel ; V edge of the inner -wall ; c c' chy- 

 miferous cavity; d f/' proboscis; e germ-basis. 

 Fig. 8. A male medusa. a inner wall of pedicel; b 

 outer wall of disk ; c inner wall of di,sk ; d proboscis ; 

 e edge of inner wall; / spermatic mass. 

 Fig. 9. A partially developed female medusa, a disk ; 

 «' outer, and 6' inner wall of pedicel; c chymiferous 

 cavity; </ proboscis; f/' tip of d\ e germ-basis; / 

 tentacles beginning to bud. 



Fig. 9a. A portion (i) of the germ-basis. o walls of 

 the disk. 



Fig. gb. Cellules of fig. 9a, 6, isolated, a wall of the 

 cell ; b contents. 



Fig. 10. A female medusa. a disk; n' outer wall of 

 pedicel; b inner wall of pedicel; c chymiferous cav- 

 ity ; (/ proljoscis ; d} tip of f/ ; e germ-basis ; / ten- 

 tacles. 



Fig. 11. A female medusa. a a> b c d as in fig. 10; 

 e a young hydroid; / the cavity of the disk; g the 

 germ-basis. 



Fig. 12. a a" b c d d^ f as in fig. 10; e e' e^ e' young 

 hydroids. 



Fig. 12a. One of the tentacles of fig. 12, seen in pro- 

 file. «' the disk; b 6' b- the entrance to the cavity 

 of the tentacle ; d outer wall ; e inner wall. 



Fig. 12b. Edgewise view of fig. 12a, looking along the 

 line a .... c. a the disk ; 6 cavity of the tentacle ; 

 e corresponds to e in fig. 12a. 



Fig. 12<^. The edge of the disk of a female medusa 

 with ten tentacles c c'. a walls of the disk ; h aper- 

 ture of the disk. 



Fig. 13. The lettering as in fig. 12, excepting e"; the 

 digestive cavity of a young hydroid, and e^, tentacles. 



Fig. 14. A medusa upon the point of discharging a 

 young hydroid. a disk; «' outer, and b^ inner wall 

 of pedicel; c proboscis of the hydroid; c' stem of 

 of the hydroid; d proboscis of the medusa; </' chy- 

 miferous cavity; e tentacles of the hydroid; //' ten- 

 tacles of the medusa; g globose tips of e. 



Fig. 14a. The stem of the hydra of fig. 14, to show the 

 horny sheath, c c' c": a outer, and h inner wall; d 

 chymiferous cavity. 



Fig. 15. A male medusa. Letters as in fig. 2a, and e 

 aperture of the disk ; </' proboscis projecting through e. 



Fig. 15a. A portion (b) of the spermatic mass of fig. 

 15. a the walls of the disk. 



Fig. 16. Spermatic particle from a mature medusa. A, 

 diagrammic, to show its form ; B as seen with 500 diams. 



