226 HYDROID^. Part IV. 



longation of the horny sheath {Figs. 17, A, to 21, c) ; but here it has much more 

 consistence than in Coryne. Each medusa is elevated on a short stem [Fig. 17, 

 A a), which elongates with age, until, by the time the spermatic particles are 

 discharged {Fig. 21), it nearly equals the length of the medusa. This stem has 

 a double wall (« b), like that of the hydroid, but the inner one {h) retreats from 

 the outer one {a) at the base of the medusa, and projects freely into the disk, as 

 a proboscis {d). In reality, although the form is altogether different, the structure 

 of the medusa is the same as in Clava ; the spermatic mass occupies a homo- 

 logous position, and is developed in the same manner, and with a similar dimi- 

 nution in the size of the proboscis. The youngest medusa which we have observed, 

 was nearly cylindrical in form {Fig. 18), being slightly swollen toward the base, 

 and coming to a jjoint rather suddenly at the end, where the wall was very thick. 

 The cylindrical proboscis {d) traversed nearly the whole length of the disk, and 

 was comjjletely enveloped in the mass of deep orange-coloi'ed spermatic matter 

 {b^), which filled the cavity of the disk. With increasing age, the spermatic mass 

 grows paler, and when fully matured, it is white {Fig. 20). After the discharge 

 of the spermatic particles, the medusa becomes cylindrical {Fig. 21), whilst the 

 proboscis dwindles down to a shrivelled, diminutive mass. 



In other genera we have been accustomed to see the medusa wither and 

 decompose, after it had matured and discharged its reproductive contents ; but 

 here an imusual and unexpected jihenomenon takes place ; one and ihe same indi- 

 vidual medusa, after discharging its reproductive organs, is metamorphosed into a Iigdra ; 

 the same Avail which formed the disk {Fig. 22, «') of the medusa grows upward 

 (a), and forms a long, cylindrical body, Avithin Avhich an inner wall {b^ b) develops, 

 from the base of the still persistent proboscis {d), and completely lines the outer 

 wall. We have traced this metamorphosis up to the time Avhen the head of the 

 hydra had begun to form, and its tentacles Avere just far enough advanced to give 

 it a knotted appearance, but mifortunately the sjjecimens died, and Ave have not 

 been able to investigate the matter any further. The figure Avhich we giA'e here, 

 representing this stage of groAvth, was taken from the animal Avhen the basal part, 

 or the original stem of the medusa, was so retracted that the base of the proboscis 

 {d) was brought nearly down to the stolon. The spermatic particles have a broadly 

 fusifoi-m head {Fig. 23, A B), and a tail only four times longer than the head. 

 This retrograde metamorphosis of a medusa into a hydra, is the most direct evidence, 

 thus far obtained, of the structural identity of the free Medusse and the Hydroids 

 proper. It shows beyond the ^possibility of a doubt that the Hydroids themselves 

 are not Polyps, but Acalephs, in the same Avay as Myriapods are Insects and not 

 Worms, notwithstanding their many rings and elongated form. 



