212 NANOMIA CARA. 



Hydroids, as in Hydractinia, only these Medusse never separate from 

 the original parent-stock from which they are produced. The float 

 corresponds to the part of the stem of the fixed Hydroids by which 

 they are attached, and the different individuals arising from this single 

 Polyp are Medusae. To show the Medusa character of these individ- 

 uals, I must preface by saying that the tentacles, the Polyp, and the 

 scale are not so many independent individuals, but that these three 

 together form one individual, the Medusa. It is true this will appear, 

 at first sight, rather doubtful ; we are accustomed to associate with 

 our notion of Medusa a regularly-shaped bell, chymiferous tubes, and 

 a proboscis. But this is not the universal character of Medusse ; the 

 abortive sessile Medusas of Sertularians, Campanularians, and Tubula- 

 rians are as much Medusae, though they have no prol)oscis and nothing 

 but rudimentary chymiferous tubes, as a free-swimming Sarsia or Bou- 

 gainvillia. The moment this is understood, a new light is thrown on 

 the nature of the individuals of our community. We find among the 

 Hydroids all the stages intermediate between a rudimentary Medusa, 

 as that of Laomedea or of Dynamena, and that of a symmetrical Me- 

 dusa like Sarsia. We have those in which the proboscis is quite well 

 developed, others in which the chymiferous tubes are always rudi- 

 mentary, others which are asymmetrical, as Hybocodon, and the differ- 

 ent spheromeres of which have not an equal degree of development. 

 This gives us the clew to the true understanding of the relation be- 

 tween the clusters of tentacles, the scale, and the Polyp ; let us take 

 one of these Hybocodon Medusae, cut away the two lateral spheromeres 

 of the bell, and at the same time reduce the spheromere which bears 

 the long tentacle to its minimum, that is, bring the tentacle to the 

 point of junction of the proboscis and of the chymiferous tubes, cut 

 the remaining spheromere in any shape we please, triangular or pen- 

 tagonal, and we have a Medusa identical in every respect to those of 

 our community. The cluster of tentacles corresj)onds to the single 

 tentacle, the scale with its tube corresponds to the remaining sphero- 

 mere and its chymiferous tube, while the proboscis and the Polyp are 

 identical, and perform one and the same function. The fact that there 

 is or is not a chymiferous tube extending through the scale, as in Fig. 

 338, does not invalidate this homology ; for the chymiferous tubes in 

 many Hydroid Medusas are exceedingly rudimentary, and in that case 

 it would only be carrying out for free Hydroids what has become 

 perfectly familiar to us among the fixed Hydroids. 



By reducing this proposition to a mathematical form, I may perhaps 

 give the reader a better idea of the process I am endeavoring to 

 explain, by supposing an equation of the nth. degree to represent the 

 formula of a Hydroid Medusa, the roots of which are represented by 

 the different spheromeres ; they all become unaginary in our Nanomia, 



