Development in the sea urchin, Sammekinus miliaris, third metamorphosis, normal speed, time-lapse, 1 to 96 to 1 to 360.

The sea urchin, an inhabitant of the sea bed, has a swimming lava called the echinopluteus. After formation of the axo, hydro and somatocele, the sea urchin disc is developed

through indentation of the body surface in the area of the left hydroseal. In a lava that is ripe for metamorphosis, the disc almost fills the left part of the body. The disc forms the oral side of the mature sea urchin.

It is supplemented by calcareous structures with pedicellari and spines which lie in the opposite part of the body. With the help of lash bands called epaulettes, the elder pluteus can swim through the water. The metamorphosis from pluteus to sea urchin occurs over a period of about one hour.

It can be initiated by addition of caesium chloride. The movements of the primary podia, now starting, are the first indications of the beginning of metamorphosis.

The sea urchin disc widens into a dome and tears open the amnion skin.

The five primary podia now stretch themselves slowly through the opening. During metamorphosis, cell material from the skeletal rods of the larval arms returns into the body.

The melting down of the larval body is shown in quick motion. The shell-shaped oral side takes up the complete cell material of the larva. Only the calcareous rods of the larval arms, freed of cell material, project out of the body of the larva. The pedicellarii, which are shown here on the right of the picture, already possess their full functional ability.

Apart from the outer calcareous rods, no material is lost through metamorphosis.

For clarification, the metamorphosis from pluteus to sea urchin is shown in graphical form. The elder pluteus possesses eight larval arms which are bilaterally symmetrically arrayed and stiffened by rods. These are the dorsal arch, the anterior lateral, post-oral and posterior dorsal rods.

On the left side is the disc and on the right side are the pedicellarii. The view of the right side of the pluteus, which is the apporal side of the mature sea urchin,

the symmetrical change can be seen well. At the rear end of the dorsal arch, on the right posterior dorsal and post-oral rods, lie three calcareous networks from which the genital plates 2, 3 and 5 on the apporal side of the sea urchin are later formed.

At the beginning of the metamorphosis, the shrinking of the larval body causes these three areas to come closer together. The cell material of the front part of the larval body is absorbed by the rear part. The genital plates 2, 3 and 5 come into contact with each other.

The genital plates 1 and 4 develop from three-pronged formations and grow quickly. Whilst two young spines develop alongside the pedicellarii of the genital plates 3 and 5, plates 1, 2 and 4 develop only one each.

The apporal skeleton of the young sea urchin is formed from five almost radially arrayed genital plates and the five terminal plates between them.

Finally, the double rows of thin calcareous plates, which are in ambulacral and interambulacral positions, give the sea urchin skeleton the characteristic radial symmetrical appearance. During metamorphosis, the internal structure changes also.

The sea urchin disc, which is the oral side of the sea urchin, is developed on the left side. During the shrinking of the body, the oral side of the sea urchin enlarges. Mouth and anus of the larva lose their function.

A new mouth forms between the five dental sacs. The young sea urchin, which has just completed its metamorphosis, turns itself on the former left side of the larva. To the right side of the madripaite plate lies the dorsal sac, which is the remains of the right axocele.

Whereas the extensive water vascular system is built by the left hydrosile, the right does not normally appear to have a function. The right and left somata seal form the coelomatic trunk of the sea urchin.

These structural relationships are also retained in the elder sea urchin. On the left side, the proportional relationships of the interambulacral segments are shown, and on the right, those of the ambulacral segments. The number of plates, podii, pedicillarii, and spines have increased.

From the five tooth structures, the so-called lamp of Aristotle is developed. In the center of which, a new mouth is formed. In the interambulacral area is a tooth. In the ambulacral area, a dental sac. Opposite the mouth, the anus now appears.

Once again, we will show the individual phases of metamorphosis in real life.

In the planned view of the oral side of the developing sea urchin, one can recognize melting of the larval arms. The larval body shrinks to the size of the sea urchin disc. Between the oral and aboral sides,

there is a brighter area surrounding the developing sea urchin. The young sea urchin is still transparent because the calcareous plates of the skeleton are not yet thickened. The podii are moved by the coelomic liquid pressure of the water vascular system and by their own muscle contractions.

Contractions of the water ring canal change the pressure in the water vascular system.

Calcareous networks surround the newly developing mouth. The five teeth of the sea urchin are formed in dental sacs

and they are functional shortly after the metamorphosis. The increasing calcareous deposit prevents further internal observations.

The calcareous layers are clearly visible in polarized light. The side view shows the structure of the young sea urchin in an upper-aboral and a lower-oral half.

The four-pointed young spines mark the position of the genital and terminal plates. The five-pointed radial symmetry of the sea urchin is developed from the bilateral symmetry of the larva by metamorphosis. The sea urchin is now about 2 mm in size

and grows continuously by building new skeletal plates.