358 M. Sars on the Nurse-Genus Corymorpha and its Species, 



Naturforskeres 7de Mode i Christiania/ 1856, pp. 194-201). 

 The genus Hydi^a (the well-known freshwater polype), which 

 differs from all other Hydroida by its power of moving from 

 place to place, and by the production of deciduous Hydraform 

 buds resembling the parent animal, unites, in a remarkable 

 manner, the solitary and composite Hydroida, since at one time, 

 namely when it has no young, it is solitary ; when it is proli- 

 ferous, or pushes forth buds, it represents a temporary colony, 

 which in a short time is dissolved, the hydraform young produced 

 by prolification gradually separating from the parent animal. 



From the foregoing observations it will be seen that the genus 

 Corymorpha produces Medusae of very different nature. Five of 

 the known species, namely, produce perfect, well-organized Me- 

 dusae, which separate from the nurse-animal and lead a com- 

 pletely free-swimming life, in which state only they develope 

 generative organs and propagate. They therefore prove to be- 

 long to the great group of the so-called lower Medusse (Crypto- 

 carpse, Eschsch. ; Gymnophthalmata, Forbes ; Craspedota, Ge- 

 genb.). One species, on the contrary, namely Corymojyha 

 glacialis, produces, instead of these, Medusse which are from the 

 first furnished with generative materials, but otherwise extremely 

 simple and imperfectly formed, and destined to remain sessile 

 or permanently connected with their nurse-animal until after 

 they have developed and evacuated their sexual products, when 

 they dissolve and disappear. 



We have here a new example of very similar nurse-animals 

 giving origin to a very different medusiform brood. Science 

 is not deficient in other examples. Thus, for instance, according 

 to Van Beneden, Tubularia Dumortieri, V. B., produces perfect 

 and deciduous, and T. larynx, Ell. & Sol. {T. coronata, V. B.), 

 imperfect and sessile Medusse. From my own observations also, 

 Podocoryna carnea, Sars, and P. tubularia, Sars, give origin to 

 perfect and deciduous Medusse, whilst P. Sarsii, Steenstr., and 

 P. fucicola, Sars, produce imperfect and sessile ones. Both 

 kinds sprout, in all these Hydroida, as in Corymorpha, in the 

 form of buds from the same spot in the nurse-animal ; they 

 have the same mode of development and the same form and 

 organization up to a certain stage, when those which are destined 

 to remain sessile are arrested in their development, whilst those 

 which are to lead an independent free life are developed further 

 into the perfect Medusa-type. 



These complicated relations greatly increase the difficulty of 

 classifying the Hydroid polypes according to their sexual genera- 

 tion — the method which is certainly most in accordance with 

 the principles ordinarily adopted in zoology, and which, indeed, 

 is employed by Gegenbaur in his otherwise admirable " Versuch 



