ALTERNATION OF GENERATIONS. 125 



and developing their generative elements subsequent to their 

 liberation. Even in the same species the male and female 

 apparatus are not always alike in structure. Mr. Huxley 

 remarks, that " the female organs in Athorybia are sacs con- 

 taining a single ovum, arrested, as it were, in the first stage 

 towards the medusiform condition, while the male organs 

 become completely medusiform, but are probably not de- 

 tached. In Physalia, on the other hand, the female organs 

 are free swimming medusoids, while the male organs are 

 simply pyriform sacs."* 



There is an equally great variety in the mode of origin 

 of the gamomorphic structures whether medusoids or 

 sporosacs from the polype stock. In the Hydroid group 

 they spring from papillary outgrowths of the gelatinous 

 basis either of the polypes or of the common axis of the 

 polypidom. When the papilla is prolonged into a fleshy 

 column, it is called by Allman a blastostyle, and generally 

 bears a cluster of sporosacs or medusoids. Sometimes the 

 blastostyle and polypes are naked, as in Clava and Tu- 

 bularia ; but more frequently they are surrounded by a 

 gonophore, or involucral capsule, derived from the horny 

 sheath of the polypidom, as in species of Campanularia, 

 Laomedea, axidAntennularia. In Hydr actinia and Cordy- 

 lophora we have gonophores without any blastostyle, the 

 capsule containing a single sessile sporosac. In Plumu- 

 laria cristata the gonophores occur in clusters, in peculiar 

 receptacles formed, as Prof. E. Forbes has shown, by a mo- 

 dification of the ordinary ramuscules of the zoophyte, f 



* Huxley, Medical Times, XII., 566. English Cyclop. Nat. Hist., 

 I., 38. 



f Ann. Nat. Hist., 1st Ser., XIV., 385. 



In the home-logy of these structures, it is the manubrium which ap- 

 pears to be the representative of an ordinary polype, the umbrella being 

 a superadded organ for natatory or other purposes. The simple gono- 

 phore may be taken as the homologue of the horny polyp cell, and the 

 blastostyle as a rudimentary polypiferous ramuscule. The compound 



