Fig. 33. Idyia roseola seen from the broad side, half natural size; a anal opening, b lateral tube, c circular tube, d e f g h rows of locomotive flappers. (Agassiz.)
The lowest genus of Ctenophoræ found on our coast, the Idyia ([Fig. 33]), has neither the tentacles of the Pleurobrachia, nor the lobes of the Bolina. It is a simple ovate sphere, the interior of which is almost entirely occupied by an immense digestive cavity. It would seem that the reception and digestion of food is intended to be the almost exclusive function of this animal, for it has a mouth whose ample dimensions correspond with its capacious stomach. Instead of the longitudinal split serving as a mouth, in the Bolina and Pleurobrachia, one end of the body in the Idyia is completely open ([Fig. 33]), so that occasionally some unsuspicious victim of smaller diameter than itself may be seen to swim into this wide portal, when suddenly the door closes behind him with a quick contraction, and he finds himself a prisoner. The Idyia does not always obtain its food after this indolent fashion however, for it often attacks a Bolina or Pleurobrachia as large or even larger than itself, when it extends its mouth to the utmost, slowly overlapping the prey it is trying to swallow by frequent and repeated contractions, and even cutting off by the same process such portions as cannot be forced into the digestive cavity.
The general internal structure of the Idyia corresponds with that of the Bolina and Pleurobrachia; it has the same tubes branching horizontally from the main cavity, then ramifying as they approach the periphery till they are multiplied to eight in all, each of which gives off one of the vertical tubes connected with the eight rows of locomotive flappers. Opposite the mouth is the eye-speck, placed as in the two other genera, at the centre of a small circumscribed area, which in the Idyia is surrounded by delicate fringes, forming a rosette at this end of the body. These animals are exceedingly brilliant in color; bright pink is their prevailing hue, though pink, red, yellow, orange, green, and purple, sometimes chase each other in quick succession along their locomotive fringes. At certain seasons, when most numerous, they even give a rosy tinge to patches on the surface of the sea. Their color is brightest and deepest before the spawning season, but as this advances, and the ovaries and spermaries are emptied, they grow paler, retaining at last only a faint pink tint. They appear early in July, rapidly attain their maximum size, and are most numerous during the first half of August. Toward the end of August they spawn, and the adults are usually destroyed by the early September storms, the young disappearing at the same time, not to be seen again till the next summer. It is an interesting question, not yet solved, to know what becomes of the summer's brood in the following winter. They probably sink into deep waters during this intervening period. The Idyia, like the Pleurobrachia, moves with the mouth upward, but inclined slightly forward also, so as to give an oblique direction to the axis of the body.[4]
[4] Until this summer only the three genera of Ctenophoræ above mentioned were supposed to exist along our coast, but during the present season I have had the good fortune to find two additional ones. One of them, the Lesueuria, resembles a Bolina with the long lobes so cut off, that they have a very stunted appearance in comparison with those of the Bolina. The other, the Mertensia, is closely allied to Pleurobrachia; it is exceedingly flattened and pear-shaped. This species was discovered long ago by Fabricius, but had escaped thus far the attention of other naturalists. (A. Agassiz.)
EMBRYOLOGY OF CTENOPHORÆ.
All the Ctenophoræ are reproduced from eggs, these eggs being so transparent that one may follow with comparative ease the changes undergone by the young while still within the egg envelope. Unfortunately, however, they are so delicate that it is impossible to keep them alive for any length of time, even by supplying them constantly with fresh sea-water, and keeping them continually in motion, both of which are essential conditions to their existence. It is therefore only from eggs accidentally fished up at different stages of growth that we may hope to ascertain any facts respecting the sequence of their development. When hatched, the little Ctenophore is already quite advanced. It is small when compared with the size of the egg envelope, and long before it is set free, it swims about with great velocity within the walls of its diminutive prison ([Fig. 35]). The importance of studying the young stages of animals can hardly find a better illustration than among the Ctenophoræ. Before their extraordinary embryonic changes were understood, many of the younger forms had found their way into our scientific annals as distinct animals, and our nomenclature thus became burdened with long lists of names which will disappear as our knowledge advances.