| Fig. 102. Unsymmetrical free Medusa of Hybocodon; r o
chymiferous tubes, v proboscis, s circular tube, m young Medusæ at base of long tentacle t. (Agassiz.) |
| Fig. 103. Medusa bud of Hybocodon; a base of attachment,
o proboscis, c circular tube, d young Medusæ at base of long tentacle t. (Agassiz.) |
| Fig. 104. Single head of Hybocodon Hydroid; a stem, d
Medusæ buds, o tentacles round mouth. (Agassiz.) |
Dysmorphosa. (Dysmorphosa fulgurans A. Ag.)
Besides the budding at the base of the tentacle, as in Hybocodon, we find another mode of development among Hydroid Medusæ, viz. that of budding from the proboscis. One of our most common little Jelly-fishes, the Dysmorphosa ([Fig. 105]), to which we owe the occasional blue phosphorescence of the sea, so brilliant at times, buds in this manner. [Fig. 105] represents an adult Dysmorphosa, on the proboscis of which may be seen three small buds in different stages of development. In [Fig. 106] the proboscis is more enlarged, showing one of the little Jelly-fishes similar to the parent, just ready to drop off. We need not wonder at the immense number of these animals, with which the sea actually swarms at times, when we know that as fast as they are dropped, and it takes but a few days to complete their development, they each begin the same process; so that in the course of a week or ten days one such Medusa, supposing it to have produced six buds only, will have given rise to forty-two Jelly-fishes, thirty-six of which may be equally prolific in the same short period. These Medusæ budding thus, and swimming about, carrying their young with them, bear such a close resemblance to the floating communities of Hydroids formerly known as Siphonophoræ, that did we not know that some of them arise from Tubularians, it would be natural to associate them with the Siphonophoræ.
| Fig. 105. Dysmorphosa seen in profile; magnified. | Fig. 106. Magnified proboscis of Dysmorphosa with young Medusæ budding from it. |
Nanomia. (Nanomia cara A. Ag.)
The Nanomia ([Fig. 115]), our free floating Hydroid, consists, when first formed, of a single Hydra containing an oblong oil bubble ([Fig. 107]). The whole organisation of such a Hydra is limited to a simple digestive cavity; it has, in fact, but one organ, and one function, and consists of an alimentary sac resembling the proboscis of a Medusa ([Fig. 107]); the oil bubble is separated from it by a transverse partition, and has no connection with the cavity. Presently, between the oil bubble and the cavity arise a number of buds of various character ([Fig. 108]), which we will describe one by one, beginning with those nearest the oil bubble, since these upper members of the little swimming community bear a very important part in its history. The infant community ([Fig. 108]) passes rapidly into the stage represented in [Fig. 109], and then through all the stages intermediate between this and the adult, shown in its natural size in [Fig. 115]. The upper buds enlarge gradually, and soon take upon themselves a perfect Medusa structure ([Fig. 110]), with the exception of the proboscis, the absence of which is easily understood, when we find that these Medusæ, serve the purpose of locomotion only, having no share in the function of feeding the community, so that a digestive apparatus would be quite superfluous for them. In every other respect they are perfect Medusæ, attached to the Hydra as the Medusa buds always are when first formed, having the (four) chymiferous tubes, characteristic of all Hydroid Medusæ, radiating from the centre to the periphery; two of these tubes are very winding, as may be seen in [Fig. 110], while the other pair are straight. The Medusæ themselves are heart-shaped in form, depressed at the centre of the upper surface, and bulging on either side into wing-like expansions, where they join the stem. These expansions interlock with one another, crossing nearly at right angles. The Medusæ-like buds are the swimming bells; by their contractions, alternately taking in and throwing out the water, they impel the whole community forward, so that it seems rather to move like one animal, than like a combination of individuals.