DEVELOPMENT. 95 



zoologist to that of a cuttlefish with four arms. Taking for granted that there is here no error 

 of observation, the obvious interpretation is that Van Beneden's hydroid affords an example of 

 development from an actinula instead of a plannla. 



This is a very important character, and one which, notwithstanding the general resemblance 

 of the trophosome to that of a Cori/ne, must remove the hydroid into a new genus, to which the 

 name <A Aclinogonium may be given. 



3. Sipiifcance of the Medusa in the Life-Series of the Hydroid. 



In our attempts to determine the significance of the sporosac, and the part it plays in the 

 life of the hydroid, no difficulty is encountered, for its entire history, from its origin to the fulfil- 

 ment of the purpose it is destined to serve in the economy of tlie hydroid, passes uninterruptedly 

 before our eyes, and proves it to be a true generative zooid, giving origin in some cases to 

 spermatozoa, in others to ova, whose development, as we have just seen, may be followed, and we 

 are thus enabled to trace back the hydroid in an unbroken series through the egg from which it 

 is developed, and the sporosac in which this egg originates, to the hydroid trophosome from 

 which the sporosac buds. 



The cases in which a similarly unbroken chain can be traced back through the free generative 

 bud or planoblast are naturally far less frequent, for in the majority of cases the planoblast 

 does not produce its generative elements until a considerable time after it has become free, and 

 has undergone more or less change of form as it continues to develop itself in the open sea ; and 

 it is very seldom that we can succeed in rearing the free medusa, in the confinement of om- tanks, 

 up to the period when it shall attain to sexual maturity, either directly, as in the gonocheme, or 

 indirectly, as in the blastocheme.^ We thus, then, almost always lose absolute evidence of identity 

 in both gonocheme and blastocheme, when presented at two distant periods of their lives ; and 

 there is in such cases, necessarily, an interruption in the series of continuous observations. 



Some uninterruptedly continuous observations, however, have been made, and we now know 

 of various instances in which the generative elements have been detected, either in the walls of the 

 manubrium or in special sexual buds developed from the gastro-vascular canals, in medusaj which 

 have been themselves traced to hydriform trophosomes ; while in others, though the free medusa 

 in which the eggs or spermatozoa have been found have not been traced by direct observation to 

 a trophosome, their resemblance to forms which have been so traced is so close as to justify us in 

 assigning to both a similar origin.^ There thus remains no longer any doubt that the significance 

 of the medusa in the life-series of the hydroid is in all essential points identical with that of the 

 sporosac ; and the assertion here made applies to both gonocheme and blastocheme, with this 

 difierence alone, that in the latter the generative elements are not produced directly, but only 



' lu the Siphonophora the opposite condition is prevalent ; for here the gonopbores, even such as 

 present tlie more complete medusal or phanerocodonic form, usually become loaded with ova or sperma- 

 tozoa before they detacb themselves from the trophosome. 



" I liave elsewhere brought together all the known instances in which medusie, whether gono- 

 phores or blastochemes, traceable to trophosomes, have been observed to develop generative elements. 

 See 'Report on the Reprod. Syst. in the Hydroida,' p. 411, &c. 



