124 COMPARATIVE ANATOMY. 



Hydroid stock (stolon). Its second stage of development. 

 It may be free, as in Nanomia, or fixed, as in Eucope. 



Hydra. A sexual bud (so called from its resemblance to 

 the genus Hydra) attached to the hydroid stock. 



Medusa bud (special gemma). A bud springing from 

 the hydroid stock, possessing the form of the Medusae.* It 

 may be permanently asexual, as in Nanomia, or possess 

 power of developing into sexual form, as in Eucope. It may 

 remain attached to hydroid, as in Nanomia, or become 

 detached, as in Eucope. Occasionally successive medusa 

 buds arise from it by gemmation, as in Coryne. 



Calyx. A capsule developed in the axils of the branched 

 hydroid stock, having for contents medusa buds, as in Cam- 

 panuhria, or male or female organs, as in Cordylophora. 



Hydroids pass the earlier stages of their existence as 

 little shrub-like communities, as in Eucope, or remain in 

 that condition through life, as in Cordylophora. Ilydrae 

 are transformed into jelly-fishes, which, in some instances, 

 break off when mature, and swim away as free animals, as 

 in Eucope, while in others, as in Laomedca, they remain per- 

 manent members of the hydroid stock, never assuming a 

 free mode of life. All these buds, when mature, whether 

 free or fixed, lay eggs in their turn, from which a fresh stock 

 arises to renew the singular cycle. (E. C. & A. Agassiz.) 



In Nanomia the planula is of an elliptical shape without 

 cilia, and contains an oval chamber filled with an oily fluid 

 which buoys the embryo. The planula develops directly into 

 a short hydroid stock, the oil chamber being retained. Three 

 kinds of hydrae are evolved from the stock, as follows: (1) An 

 open proboscidiform bud, with numerous long and delicate 

 tentacles springing from its base, each knobbed at the end 

 and paved with lasso cells. (2) Similar to the preceding ex- 

 cepting tentacles are short and twisted. (3) A closed hydra 

 with a single pendent tentacle of moderate length. The first 

 two forms are held to be feeders to the stock; the function 



* 'Medusa,' a genus of Discophora, is here employed in the plural to 

 include a variety of medusiform zooids. 



