(2) 



EXPLANATION OF THE PLATES. 



of either hydra or medusa; a" t]ie thickoned oral end 

 of the disk ; h and i' inner wall of the same ; Ir 

 the spermatie mass ; c' and c^ the horn-like sheath ; 

 d proboscis of the medusa; e and e' cliymiferous canal 

 or cavity ; / stolon ; m mouth of the hydi'a ; ; tentacles. 



Fig. 17. A hydra (13) and a young medusa (A) arising 

 from the same stolon. 



Fig. 18. A very young medusa, with a large proboscis. 



Fig. 19. A half ripe medusa, with the proboscis expanded. 



Fig. 20. A ripe medusa, with a shrivelled proboscis. 



Fig. 21. A medusa which has discharged its spermatic 

 particles. 



Fig. 22. A medusa metamorphozing into a hydra. 



Fig. 23. Spermatic particle of the medusa of fig. 20. A 

 is magnified 500 diameters ; B is exaggerated, to show 

 the form. 



PLATE XXI. 



Clava leptostyla Aij. 



[Figs. 1, D, and fig. 3, from n.iture, by II. J. Clark; the others by 

 A. Sonrel.J 



All the figures are lettered con-espondingly. a the ten- 

 tacles ; h the medusa; ; c the head of the hydra ; d 

 the slender base of the hydra; c the stolon; / the 

 outer, and /' the inner wall ; g the digestive cavity 

 or chymiferous canal ; </' the mouth ; n the pedicel of 

 the bunch of medusaB ; jy the proboscis of the medusic. 



Fig. 1. A hydromedusarium attached to a sea-weed. 

 Natural size. 



Fig. 2. A hydromedusarium, magnified to show the various 

 forms and attitudes of the individual hydra.', A to II. 

 25 diameters. 



Fig. 3. A young hydra, just commencing to bud. (JO 

 diameters. 



Fig. 4. A young hydra, with very few tentacles. CO diams. 



Fig. 5. A young hjdra, transversely wrinkled by con- 

 traction. GO diameters. 



Fig. G. A young hydra, having nine or ten tentacles, 

 with the mouth wide open. GO diameters. 



Fig. 7. A young hydra, with no more tentacles than 

 that of fig. 6, but much larger. GO diameters. 



Fig. 7*. View of fig. 7 from above, the mouth wide open. 

 GO diameters. 



Fig. 8. l^Iouth and upper tentacles of a full-grown hydra, 

 showing the proboscis reverted. 80 diameters. 



Fig. 8a. A single bunch of medusas from fig. 8. 80 

 diameters. 



Fig. 9. The same as fig. 8, but strongly contracted. 

 200 diameters. 



Figs. 10 and lOi. The young hydra or plannla, just 

 escaped li-cini the medusa, and swimming about by 

 means of vibratik- cilia. 200 diameters. 



PLATE XXIL 



Figs. 1-20, TlIAMNOCXIDIA SPECTABILIS Ag.'^ Figs. 21-30, 

 T. TENELL.\ A(J. 



[Figs. 1-15 and 17, drawn by H. J. Clark; the others by 

 A. Sonrel.] 



In figs. 1 to 14, a outer wall of the medusa; a? outer 

 wall of the pedicel of the medusa ; J inner wall of 

 the medusa; i' inner wall of the pedicel; c chy- 

 miferous cavity ; (/ proboscis ; d} proboscis seen through 

 the young hydroid ; e germ basis ; e' young hydroids ; 

 e- cavity of the disk ; f tentacles ; si basal end of 

 the hydroid ; p proboscis of the hydroid ; te tentacles 

 of the hydroid. 



These figures (1-15) represent the origin and mode of 

 gi'owth of the medusa and the hydroids which it con- 

 tains. Figs. 1, 4, G, C", 7, 8, 9, 10, 11, 12, 13, and 

 14, are magnified 100 diameters ; figs. 2, 3, and 5, 

 300 diameters; figs. 4" and 8% 200 diameters; fig. 15, 

 GO diameters. August, 1831. 



In figures 15 to 30, excepting when stated otherwise, a 

 is the base of the proboscis ; h the oral end of the 

 proboscis ; /;' the top of the stem ; c the inner mar- 

 gin of the open mouth; d the medusa; ; </' young 

 medusa;-buds ; c medusifcrous branches ; p proboscis ; 

 p' decurrent bases of proboscidal tentacles ; t coronal 

 tentacles; I' proboscidal tentacles; fi branching coro- 

 nal tentacle; /* aperture of proboscis. 



Fig. 15. A young hydroid just set free. CO diameters. 



Fig. IC. Hydromedusarium of T. spectabilis. Natural size. 



Fig. 17. A bunch of female medusje, in different stages 

 of gi-owth. 25 diameters. 



Fig. 18. End view of the proboscis. 



Fig. 18H. Profile view of fig. 18. 



Fig. 19. Birds-eye view, showing the gaping mouth and 

 the constricted proboscis. 



Fig. 20. The jiroboscis enormously distended. 



Fig. 21. Hydromedusarium of T. tenella. Natural size. 

 a the new branches; h the stems of the individual 

 hydroids. 



Figs. 22 to 30. Magnified 25 diameters. 



Fig. 22. Shows the coronal tentacles, contracted so as 

 to appear globose at the tip. 



Fig. 23. Birds-eye view, to show the ciixde of medu- 

 sifcrous branches around the proboscis. 



