The Echinodermata in most cases “go through a very well-marked metamorphosis, which often has more than one larval stage.... The mass of more or less differentiated sarcode, of which the larva, or pseud-embryo, as opposed to the Echinoderm within it, is made up, always carries upon its exterior certain bilaterally-arranged ciliated bands, by the action of which the whole organism is moved from place to place; and it may be strengthened by the super-addition to it of a framework of calcareous rods.”[29] Müller considered that the mouth and pharynx of the larva were either absorbed or cast off with the calcareous rods, but were never converted into the corresponding organs of the perfect Echinoderm. According to A. Agassiz, however, this is not the case, but on the contrary “the whole larva and all its appendages are gradually drawn into the body, and appropriated.”[30] Fig. 39 represents the larva of a sea-egg (_Echino cidaris_) after Müller.[31] The body is transparent, shaped somewhat like a double easel, but with two long horns in front, which, as well as the posterior processes, are supported by calcareous rods. This larva swims by means of minute vibratile hairs, or ciliæ. It has a mouth, stomach, and in fact a well-defined alimentary canal; but no nerves or other internal organs have yet been discovered in it. After swimming about in this condition for a while, it begins to show signs of change. An involution of the integument takes place on one side of the back, and continues to deepen till it reaches a mass or store of what is called blastema, or the raw material of the animal body. This blastema then begins to change, and gradually assumes the form of the perfect Echinoderm.[32] [Illustration: FIG. 39.—Larva of _Echino cidaris_, seen from above × 6/10 (after Müller).] [Illustration: FIG. 40, Larva of _Echinus_, × 100. _A_, front arm; _F_, arms of the mouth process; _B_, posterior side arm; _E_₁, accessory arm of the mouth process; _a_, mouth; _a´_, œsophagus; _b_, stomach; _b´_, intestine; _o_, posterior orifice; _d_, ciliated bands; _f_, ciliated epaulets; _c_, disc of future _Echinus_ (after Müller).] Fig. 40 represents a larva, probably of another sea-egg (_Echinus lividus_), from the Mediterranean, and shows the commencement of the sea-egg within the body of the larva. The capital letters denote the different arms: _a_ is the mouth, _a´_ the œsophagus, _b_ the stomach, _b´_ the intestine, _f_ the ciliated lobes or epaulets, _c_ the young sea-egg.