(22) 



EXPLANATION OF THE PLATES. 



and li inner wall of the livdra ; '/ digestive cavity ; 

 I lasso-cells. \w\ diameters. 



Fig. .'*. A two-thirds grown me<Iusa. a h d I as in 

 fif. 7 ; //' inner wall of the medusa ; h° point of 

 transition of U into the wall of the ]iroliosci.s c ; 

 e cavity of the disk, containing the sperniatii.' mass. 

 400 diameters. 



Fig. 0. A i-ipe medusa, a outer h inner wall of the 

 peduncle ; //- as in fig. 8 ; c proboscis ; (/ digestive 

 cavity ; u the spermatic mass. 



Fis;. ;i^. Spermatic particles from fig. 9 ; A is magni- 

 fied ."joO diameters; B an exaggerated figure to show 

 the form of A; li the so-c:dled head; ( the fila- 

 ment. 



Fig. 10. Lasso-cells fi-om the medusa; of fig. 4. 50(.i 

 diameters. 



Fig. 11. Lasso-cells from fig. i^. a a closed cell; b a 

 cell with the thread (c) out; ?/ the base of c. 800 

 diameters. 



PL.\Tr.s XVn., XVIII., and XIX. represent the structure 

 and growth of one of the most common Ilydroids of 

 the Bay of Boston, and the mode of growth and 

 structure of its medusa, which I have already described 

 in my first paper on the Acalephs of Xorth America, 

 under the name of Sarsia mirabilis. 



PLATE XVII. 



C0RY>'E MIRABILIS Aff. 



[Figs. 1, la, 3. 4, 5, G, 7, 8, 9, 10. and 11a drawn liy A. Sonrel; the 

 others by II. J. Chirk.] 



Unless when stated otherwise, the following letters refer 

 to the same parts in all the figures. (/ inner wall 

 of the livilra ; h outer wall ; c horn-like sheath ; en 

 top of the stem ; (/ (/' digestive cavity of the stem 

 and head of the hydra ; dc disk of the medusa ; m 

 mouth of the hydra; nul medusa buds: n proboscis; 

 2) peduncle of the medusa; p'' transverse veil of the 

 medusa ; r tentacles of the medusa ; s stem of the 

 hvdra ; / tentacles of the hydra. 



FIl'. 1. A gi'oup of hydraj attached to a sea-weed. It 

 being the beginning of the breeding season (January 

 31, 185.5), the young medusa buds are not conspicu- 

 ous. X'atural size. 



Fig. I'l. A portion of fig. 1 magnified about 20 diame- 

 ters, a a very yotmg hytlra bud. 



Fig 



Fig 



. 2. A single individual, showing that the medusa? 

 are sometimes developed among the tentacles (see md). 

 m other medusfe below the tentacles. 40 diameters. 

 3 to 8. Show the various ages and attitudes of 

 the hydra. a n' medusK buds in different stages of 

 growth. 40 diameters. 



■. 3. A head of a hydra, contracted, showing the 

 horn-like sheath (c) separated from the neck. 100 

 diameters. 



:. 10. A group of hydro-medusa; late in the breeding 

 season (April 25, 1855), when the heads arc resorbing 

 and the medusa; are prominent. (See figs. 11, 12, 

 13, 14, and 15.) Xatural size. 



;. 11. A male hydra from fig. 10, the medusa per- 

 sistent, and developing the spermatic mass around the 

 jjroboscis (;/) to an enormous extent. <10 diameters. 

 . ll''. View of fig. 11 from the actiual end. 

 ■. 1 2. A male hydra from fig. 1 ; the almost per- 

 fect medusa is persistent and withering, having dis- 

 charged its spermatic contents. </ jicduncle of the 

 medusa. 40 diameters. 



;. 13. Similar to tig. 12, but the tentacles of the 

 hydra have begun to be resorbed. The medusa is 

 proportionately larger, and has no tentacles. 40 

 diameters. 



r. 14. The head of the hycba is nearly all resorbed, 

 and the medusa, without tentacles, is withering, having 

 discharged its spermatic particles. 40 diameters. 

 ;. 15. The head of the hydra, a female, is altogether 

 resorbed, and the medusa terminates the stem, like a 

 head. o' the radiating cauak, of which there are 

 five. CO diameters. 



r. IG. A female medusa attached to a hydra, and the 

 proboscis enormously distended and crowded with eggs. 

 40 diameters. 



PLATE XVIII. 



CORYXE MIR.\BILIS Ay. 

 [.Vll the tjguros are drawn from nature by H. J. Clark.] 



Fic;s. 1 to 12 are magnified 400 diameters. 



Fis. 1. A medusa just beginning to bud. a inner and 



h outer wall of hydra; c inner and h outer wall of 



the bud. 

 Flo. 2. The medusa bud already semi-globular, n outer 



and h inner wall of the hydra ; c inner and d outer 



wall of the medusa; e e' chj-miferous cavity leading 



into the medusa. 



