212 NANOMIA CARA. 



Hjdroid.s, as in Hj'dractinia, only these Medusas never separate from 

 the original parentrstock fiom which the}^ are produced. The float 

 corresponds to the pai't 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 chax'acter of these indi\id- 

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

 scale are not so miiny 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 univei'sal character of Medusa? ; the 

 abortive sessile Medusas of Sertularians, Campanularians, and Tu1)ida- 

 riaus are as much Medusa?, though they have no proboscis 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 

 Hydi-oids 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 Medusa?, 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 corresponds 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 Ilydroid Medusas are exceedingly rudimentary, and in that ca.se 

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

 perfectly familiar to us among the fixed Hydi'oids. 



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 ?*th degree to represent the 

 li)nnula ol' a llydruid Medusa, the roots of which are represented liy 

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



