As this process continues, the little creature gradually loses its power of swimming, and, sinking to the bottom, looses the bands of ciliæ, and attaches itself by its base to some stone or other solid substance, the knob of the club being free. The calcareous framework increases in size, and the expanded head forms itself into a cup, round which from five to fifteen delicate tentacles, as shown in Fig. [44], make their appearance.
In this stage the young animal resembles one of the stalked Crinoids, a family of Echinoderms very abundant in earlier geological periods, but which has almost disappeared, being, as we see, now represented by the young states of existing more advanced, free, species. This attached, plant-like condition of Comatula was indeed at first supposed to be a mature form, and was named Pentacrinus; but we now know that it is only a stage in the development of Comatula. The so-called Pentacrinus increases considerably in size, and after various gradual changes, which time does not now permit me to describe, quits the stalk, and becomes a free Comatula.
The metamorphoses of the Starfishes are also very remarkable. Sars discovered, in the year 1835, a curious little creature about an inch in length, which he named Bipinnaria asterigera (Figs. [45-47]), and which he then supposed to be allied to the ciliograde Medusæ. Subsequent observations, however, made in 1844, suggested to him that it was the larva of a Starfish, and in 1847 MM. Koren and Danielssen satisfied themselves that this was the case.
Figs. [45 and 46] represent the front and side view of a Bipinnaria found by Müller[34] near Marseilles. a is the mouth, b the œsophagus, c the stomach, c´ the intestine. Fig. [47] represents a somewhat older specimen, in which the Starfish (k) is already beginning to make its appearance.
Fig. 45, Larva of Starfish (Bipinnaria), ✕ 100 (after Müller). 46, Larva of Starfish (Bipinnaria), ✕ 100, seen from the side—a, mouth; b, œsophagus; c, stomach; c´, intestine. 47, Larva of another Bipinnaria, showing the commencement of the Starfish—g, canal of the ciliated sac; i, rudiments of tentacles; d, ciliated band.
But while certain Starfishes thus go through metamorphoses similar in character, and not less remarkable than those of sea-eggs, there are others—as, for instance, the genus Asteracanthion—in which development may be said to be direct—the organs and appendages special to the Pseudembryo being in abeyance; while in another genus, Pteraster, they are reduced to a mere investing membrane.[35]