EXPLANATION OF THE PLATES. 



(25) 



Fig. 15. A medusa just (Impppil from tlie liyilra. Feb. 

 14, 1855. Natural size. 



Fig. 15^. The same as fig. 15, nuignificil about 40 tliame- 

 ters. a remains of the chymiferou.s eliannel of the pe- 

 duncular attachment ; b outer wall of tlie proboscis ; c 

 radiating tubes ; c' circular tube ; d fold of the inner- 

 most wall ; (/' transverse fold of the iimer wall ; c aper- 

 ture of the proboscis. 



Fig. IG. A young free medusa, in a d}iiig state, com- 

 pressed and folded longitudinally. Seen from the 

 abactinal cml. (t innermost wall receding from the 

 disk (i) ; c radiating tubes; rl digestive cavity. 100 

 diameters. 



Fig. 1 7. View from the abactinal end of a medusa a little 

 older than fig. l^i"-. a the veil; h circular tube; c 

 proboscis; d digestive cavity; p innermost wall; c' 

 point of attachment of e to the disk. 100 diams. 



Fig. 18. About the same age as fig. 17, but very much 

 contracted, a longitudinal folds ; h <: corrugated lines 

 on the outer surface of the disk. 125 diameters. 



Figs. 19, 20, 21, 22, 2.'i, and 24 are all lettered alike. 

 V the vitelline sac ; jr yolk ; p Furkinjean vesicle ; 

 w Wagnerian vesicle ; vl Valentinian vesicle. 



Figs. 19, 20, 22, 23, and 24. Various stages of develop- 

 ment of the eggs of a full-grown free medusa. Jlay 

 17, 1855. 500 diameters. 



Fig. 21. An egg from fig. 15, PI. XVII. 500 diameters. 



Fig. 21a. A layer of eggs from fig. 15, I'l. XVII. « outer 

 and b inner wall of the proboscis. 400 diameters. 



Figs. 25 and 25». Spermatic particle of a full-grown free 

 medusa. Fig. 25, 500 diameters ; fig. 25» exaggerated, 

 the better to show the form. 



PLATE XIX. 



C'OKVXE MIHADILIS Af/. 

 [All the figures .-ire drawn from nature by II. J. Clark.] 



Fig. 1. A portion of the boily and a tentacle of a hydra, 

 showing the furrows g <f in the outer wall b U. 

 f globular mass of lasso-cells. 500 diameters. 



Fig. 2. Portion of the body and a sectional view of a 

 partially extended tentacle, a outer wall of the body 

 in profile ; a' the same as a, in a full view ; a- o' n' 

 cells of the inner wall of the tentacle; li outer wall 

 of the body; J' outer wall of the tentacle; c horn- 

 like sheath ; d outline of the digestive cavity ; e space 

 between the outer and inner walls of the tentacle ; 



f layer of lasso-oells at the tip of the tentacle ; // 

 processes around the mesoblast of the cells of the tenta- 

 cle. 400 diameters. 



Fig. .3. Sni-face view of a tentacle. a h cells of the 

 inner wall ; '■ outer wall ; d c- y profile of cell walls 

 of a h \ f globidar mass of tentacles. 300 diameters. 



Fig. 4. .Sectional vic-w of the body just below the tenta- 

 cles, a inner wall ; li outer wall ; c horn-like sheath ; 

 d digestive cavity. 500 diameters. 



Fig. 5. Lasso-cell of a hydra. a wall of the cell ; h 

 //' Jr axial colunui, which corresponds to the base of 

 the lasso-thread; r the anchors; d, coil of the lasso; 

 _/' aperture. 1100 diameters. 



Fig. 5^. The same as fig. 5 uncoiled. a the empty 

 cell ; b thicker part of the base of the lasso-thread ; 

 ?/' Avhere the thread begins to taper ; r c' the anchors 

 or barbs, c' is seen through h\ d the thread; </' end 

 of the basal portion; e cavity of o; f aperture of 

 the cell. 



Fig. G. Lasso-cell fi-oni the proboscis of a full-grown free 

 medusa. a profiht of the spiral coil rf; / aperture 

 of the cell. 1 1 00 diameters. 



Fig. Ca. The same as fig. 6, but the basal portion of the 

 thread everted, a the inverted thread passing through 

 the basal jjart back to the coiled part d. 



Fig. 7. Edge of the disk and a tentacle of fig. 13, PI. 

 XVni., principally to show the cellular structure of the 

 outer wall (a') of the tentacle, and disk («) ; b wall 

 of the radiating tulje; &' inner wall of the tentacle, 

 continuous with h ; c circular canal ; d cavity at the 

 base of the tentacle ; (/' channel of the tentacle ; e 

 innermost wall of the disk. 400 diameters. 



Fig. 1°-. The outer wall of the disk of fig. 7 in profile, 

 and more highly magnified, a outer ends ; b inner 

 ends. 500 diameters. 



Fig. 7''. Superficial or end view of fig. 7a. 



Fig. 8. E\e-speck of fig. IS^i, PI. XVUI. u outer wall, 

 and (• inner wall, of the e.xterior base of the tenta- 

 cle ; w a las.so-cell. 1100 diameters. 



Fig. 8". A few oily globules from the dark mass of fig. 8. 



Fig. II. The edge of the disk and the base of a tenta- 

 cle of tlie medusa of fig. 12, PI. XVII. a outer wall 

 of the tentacle ; b circular tube ; d entrance of b Into 

 the radiating tube (c) ; e innermost wall of the disk. 

 200 diameters. 



Fig. 10. Profile section of a part of the disk and radi- 

 ating tube of a medusa about ready to drop from the 

 hydra. a wall of the tube ; // innermost wall, and 

 b^ middle wall, of the disk ; c outermost wall. 500 

 diameters. 



