318 HYDROIDiE. Part IV. 



of growth (PL XXXIII. Fk/. 6, B) a long time before the calycle is fully developed 

 (PI. XXXIV. Fi(f. 10), but are not set free nntil the latter is mature. 



Emhryology. Proles mcdusoidea. — The first steps in the development of the 

 medusa are precisely the same as in Coryne mirabilis (p. 192); the outer (PI. 

 XXXIV. Fig. 11, G, {i") and the inner (}'') wall push out from the axis and form 

 a hernia ; the hernia continues to grow until it becomes pear-shaped (PI. XXXIV. 

 Fig. 10, A), and then the radiating tubes {Fig. 16, B, h) and the proboscis {j)) 

 begin to form ; the radiating tubes, extending their extremities, finally reach the 

 actinal end of the disk {Fig. 13, 13% h) ; at the same time the tubes, or rather 

 the inner wall in which they are developed, includes, as it were in a cup, a 

 prolongation {Fig. 13, h^) of the outer wall exactly as in Coryne. 



There are differences in the proportions of the embryos of these two genera, 

 which, however, do not clash with the typical mode of development of the Hydro- 

 medusas ; at a period not long before the radiating tubes unite latei'ally to form 

 the circular tube, the embryo gradually changes from a globular to a broadly 

 discoid form {Fig. IT), and the radiating tubes {h) are correspondingly broadened, 

 but, in subsequent phases, become proportionately narrower. By the time the 

 embryo is two thirds grown the tentacles {Fig. 11, C, t) begin to bud, appearing 

 like broad i^apillje when seen in profile; and the proboscis {p) projects prominently 

 beyond the outlines of the disk. As the tentacles are developed, they curl 

 inwardly upon themselves, so that, to the very last moment before birth, they 

 appear externally as broad ci'enuliitions {Fig. 11, A, B,- t). Some time before 

 birth the mouth assumes its four-cornered, characteristic {Fig. 10, A, m) form. 

 Finally, the embryo breaks loose from its attachment Avith convulsive, systolic 

 contractions, and finds its way out of the calycle between the walls of the axis 

 and the edge of the aperture. In the very act of extrusion, it expands its disk 

 and unrolls the tentacles, so that by the time it has freed itself from the embrace 

 of its parent, it is already fully expanded, and at once commences the diasystolic 

 movements of the act of swimming. It has, at birth, sixteen tentacles (PI. XXXIV. 

 Fig. 18, t, f) and four broad radiating tubes (/^), and a circular tube (/) equally 

 broad, which are quite conspicuous, and render the observation of its movements 

 very easy. When in a state of rest it usually retains the diastolic state, the 

 tentacles are thrown upwards, and their ends droop in graceful curves, in a contrary 

 direction to the concavity of the reverted disk, while the proboscis, hanging below,- 

 adds to the resemblance of a broad vase, with herbage pendent from its edge. 

 Suddenly it revferses its position, and then the proboscis hangs from the centre of 

 a bi'oad concavity, the tentacles curving in the same direction, when the medusa 

 has altogether a drooping appearance. From one of these extremes to the other, it 

 passes, during the act of swimming, witli various degrees of rapidity ; and, at times. 



