Fig. 260. Bipinnaria larva of an Asteroid. (From Gegenbaur; after Müller.)
b. mouth; a. anus; h. madreporic canal; i. ambulacral rosette; c. stomach; d. g. e. etc. arms of Bipinnaria; A. abactinal disc of young Asteroid.
As development proceeds the abactinal surface becomes a firm and definite disc, owing to the growth of the original calcareous spicules into more or less definite plates, and to the development of five fresh plates nearer the centre of the disc and interradial in position. Still later a central calcareous plate appears on the abactinal surface, which is thus formed of a central plate, surrounded by a ring of five interradial plates, and then again by a ring of five radial plates. The abactinal disc now also grows out into five short processes, separated by five shallow notches. These processes are the rudiments of the five arms, and each of them corresponds to one of the lobes of the water-vascular rosette. A calcareous deposit is formed round the opening of the water-vascular canal, which becomes the madreporic tubercle[221]. At about this stage the absorption of the larval appendages takes place. The whole anterior part of the larva with the great præ-oral lobe has hitherto remained unchanged, but now it contracts and undergoes absorption, and becomes completely withdrawn into the disc of the future starfish. The larval mouth is transported into the centre of the actinal disc. In the larvæ observed by Agassiz and Metschnikoff nothing was cast off, but the whole absorbed.
According to Müller and Koren and Danielssen this is not the case in the larva observed by them, but part of the larva is thrown off, and lives for some time independently.
After the absorption of the larval appendages the actinal and abactinal surfaces of the young starfish approach each other, owing to the flattening of the stomach; at the same time they lose their spiral form, and become flat discs, which fit each other. Each of the lobes of the rosette of the water-vascular system becomes one of the radial water-vascular canals. It first becomes five-lobed, each lobe forming a rudimentary tube foot, and on each side of the middle lobe two fresh ones next spring out, and so on in succession. The terminal median lobe forms the tentacle at the end of the arm, and the eye is developed at its base. The growth of the water-vascular canals keeps pace with that of the arms, and the tube feet become supported at their base by an ingrowth of calcareous matter. The whole of the calcareous skeleton of the larva passes directly into that of the adult, and spines are very soon formed on the plates of the abactinal surface. The original radial plates, together with the spines which they have, are gradually pushed outwards with the growth of the arms by the continual addition of fresh rows of spines between the terminal plate and the plate next to it. It thus comes about that the original radial plates persist at the end of the arms, in connection with the unpaired tentacles which form the apex of the radial water-vascular tubes.
It has already been mentioned that according to Metschnikoff (No. [560]) a new œsophagus is formed which perforates the water-vascular ring, and connects the original stomach with the original mouth. Agassiz (No. [543]) maintains that the water-vascular ring grows round the primitive œsophagus. He says—“During the shrinking of the larva the long œsophagus becomes shortened and contracted, bringing the opening of the mouth of the larva to the level of the opening of the œsophagus, which eventually becomes the true mouth of the starfish.” The primitive anus is believed by Metschnikoff to disappear, but by Agassiz to remain. This discrepancy very possibly depends upon these investigators having worked at different species.
There is no doubt that the whole of the larval organs, with the possible exception of the œsophagus, and anus (where absent in the adult), pass directly into the corresponding organs of the starfish—and that the præ-oral part of the body and arms of the larva are absorbed and not cast off.
In addition to the Bipinnarian type of Asteroid larva a series of other forms has been described by Müller (No. [561]), Sars, Koren, and Danielssen (No. [554]) and other investigators, which are however very imperfectly known. The best known form is one first of all discovered by Sars in Echinaster Sarsii, and the more or less similar larvæ subsequently investigated by Agassiz, Busch, Müller, Wyville Thomson, etc. of another species of Echinaster and of Asteracanthion. These larvæ on leaving the egg have an oval form, and are uniformly covered by cilia. Four processes (or in Agassiz’ type one process) grow out from the body; by these the larvæ fix themselves. In the case of Echinaster the larvæ are fixed in the ventral concavity of the disc of the mother, between the five arms, where a temporary brood-pouch is established. The main part of the body is converted directly into the disc of the young starfish, while the four processes come to spring from the ventral surface, and are attached to the water-vascular ring. Eventually they atrophy completely. Of the internal structure but little is known; till the permanent mouth is formed, after the development of the young starfish is pretty well advanced, the stomach has no communication with the exterior.