(12) 



EXPLANATION OF THE PLATES. 



Fig. 5. A calycle, pedicel, and a joint of the stem, in 

 sectional profile. a a' the thickness of the calycle 

 walls ((I is one thinl too thick); e e^ the thickness 

 of the sheath ; f-y upper and lower ends of the joint 

 of the stem ; h the base of tlie pedicel ; k the semi- 

 partition ; m the deflected edge of L The arrows 

 are explained in figs. 5» and G. 100 diameters. 

 Fi". 5». An end view of fig. 5, seen as if along the 

 arrow 2, a little oblique to the axis. a the edge ; 

 c corresponds to a in fig. o ; b the tentacles ; d the 

 outskirts of the proboscis. 

 Fig. 6. A view of a terminal hydra, its pedicel and 

 the last joint of the stem, seen as if along the arrow 

 3 of fig. 5. a the calycle; b cavity of a ; d the 

 j>roboscis ; e the horn-like sheath of the stem ; g the 

 base of the joint ; /; the next joint below ij ; i outer 

 wall of the stem ; k semi-partition. 

 Fig. 7. The base of a calycle. k I m n various parts 



of the semi-partition. 500 diameters. 

 Fig. 8. A stem viewed so that the hydraj (A-F) project 



toward, and directly from, the eye. 40 diameters. 

 Fig. 9. A hydrarium creeping over a sea-weed. Nat- 

 ural size. 

 Fi"'. 9". A view of one quarter of the medusa, a the 

 eyes ; (3 the base of the tentacles ; y the lateral 

 swellings of the tentacles ; /' the circular canal ; ,/'' 

 the genital organ ; h* the base of the proboscis (p) ; 

 ( tentacles ; v the veil. 200 diameters. 

 Fig. 10. The terminal half of an immature hydromedusa. 

 /3 outer, and 7 inner wall of the axis ; A A' the 

 medusa; ; c the chymiferous channel ; d the undevel- 

 oped end of the axis ; k the calycle ; k^ the operele ; 

 k? the edge of i' ; m mouth of A ; < tentacles of A. 

 400 diameters. 

 Fig. 11. A mature hydromedusa. f3 outer, and y inner 

 wall of the axis ; /3' /3"' processes from /3 ; /3" outer, 

 and y inner wall of the young medusa (G) ; A-G 

 the medusiE ; a the axis ; d' the chymiferous channel ; 

 k the calycle ; A' the aperture of the calycle ; k- the 

 depressed base of the neck ; l^ the edge of the de- 

 pression {k°) ; p the proboscis of the medusa ; ( the 

 tentacles of the medusa. 400 diameters. 

 Fig. 12. A quarter of a medusa, at the time of birth. 

 a the eyes ; P base of the tentacles ; y lateral swell- 

 incs of < ; a outer, and h middle wall of the disk ; 

 a' outer, and 6' inner wall of the tentacles ; f cir- 

 cular tube ; /' radiating tube ; g- wall of /' ; A' mouth 

 of the proboscis {p) ; t t^ the tentacles ; v the veil. 

 400 diameters. 

 Fig. 13. A young medusa-bud in profile; h the radi- 



ating tubes, or inner wall ; A' the disk. 300 di- 

 ameters. 



Fig. 13*. An end view of fig. 13. h as before. 



Fig. 14, 14a. Cells from the outer wall of p, fig. 12. 

 500 diameters. 



Fig. 15. Cells from the lower surface of fig. 12. 500 

 diameters. 



Fig. 16. Two medusic (A B) from fig. 11. /( /i' as 

 in fig. 13 ; Ir outer, and h' inner wall of the pedicel. 

 400 diameters. 



Fig. 17. A medusa from fig. 11, in profile; h li^ If 

 ¥ as in fig. IG ; /i' chymiferous cavity. 300 diame- 

 ters. 



Fig. 18. A medusa just escaped from the calycle; seen 

 from above. Letters as in fig. 12. 300 diameters. 



Fig. 18*. A tentacle of fig 18, a in profile, to show 

 the prominence of the eye (o). 



Fig. 19. The natural size of fig. 18. 



Fig. 20. A tentacle from fig. 18, seen fi'om above, a 

 outer wall of the disk ; i' axial or inner wall of the 

 tentacle ; / circular tube ; /' radiating tube ; g g^ 

 innermost wall of the disk ; I lasso-cells ; ji centripetal 

 projection of the axial wall of the tentacle ; 7 lateral 

 swelling of the base of the tentacle. 500 diams. 



Fig. 21. An oculiferous tentacle, from fig. 18, seen from 

 below. <i /;' r/' / 7 as in fig. 20 ; a the eye ; a the 

 lenticular body of a ; v the veil. 

 Fi". 21*. The lateral swel'ing, 7, fig. 21, seen isolately. 



PLATE XXXV. 



PhYS.\LIA .\nETlIUSA Tit. 

 [Drawn from nature by A. Agassiz and A. Sonrel.] 



Fig. 1. Specimen floating quietly upon the surface of 



the water, with tentacles drooping loosely, seen from 



the windward side. 

 Fi". 2. The same specimen from the opposite side. 

 Fig. 3. Transverse section of the floating hydra, to show 



the relations of the secondary hydra; and clusters of 



hydra; and meduste. 



Obliged, on account of my eyes, to depend, in a 

 great measure, upon others for the revision of my proofs, 

 I request the reader to excuse the mistakes that may 

 have been allowed to pass unnoticed in this volume. 



Cambridge, May 1, 1S62. 



