Development of the sea urchin Sammichinus miliaris, differentiation of sea loam. The sea urchin develops from a larva, the echinopluteus.

The adult is radially symmetrical and moves by ambulacral podia on the bottom of the sea. The planktonic larva, however, has a distinct bilateral symmetry and swims in the water by the movements of its ciliated bands.

Here, in a lateral view, the young pluteus has a complete digestive system with esophagus, stomach, intestine and anus. The larva has an elongated body with four larval arms surrounding the mouth. Body and arms are stiffened by calcareous skeletal rods.

In the dorsal view, only the esophagus and stomach are recognizable. The pluteus is about four days old and is bilaterally symmetrical. This alters during the development of the sea loam, which the following diagram shows.

There are two coelomic sacs which are formed at the end of the archenteron by division of one coelomic sac. They are seen here to the left and right of the esophagus. The left coelome is larger than the right. This becomes clearer later on in its development. Both coelomic sacs move back towards the stomach

along the plasmatic extensions of the cells of the secondary mesenchyme. Simultaneously, a thin extension grows from the left coelomic sac to the dorsal side of the larva. This is the analoga of the stone canal. About seven days after fertilization, each coelomic sac divides into an anterior and a posterior sac.

The anterior parts remain at the esophagus. The posterior sacs move further along mesenchymatic strands towards the stomach.

A little later, the posterior sacs divide again. There thus appears an archimetric articulation of the coelome in axocele, hydroseal and somatoseal known as procele, mesocele and metaseal in other invertebrates. The differentiation of the left coelomic sacs

is much greater than that of the right ones. The rudimentary right axocele is later called the dorsal sac. The right hydroseal disappears and the right somatoseal remains small. Viewed from the left side, the hydroseal is in the central part of the larva.

The axocele continues beyond the stone canal to the dorsal side of the larva. In this manner, axocele and hydroseal are connected with the surface of the body. The somatoseal covers nearly the whole left side of the stomach.

From this five days old pluteus, one can see coelomic sacs on both sides of the esophagus. The left one is already larger than the right. On the eighth day, the coelome sacs divide for the first time.

The anterior parts remain near the esophagus, the posterior parts develop towards the stomach. They divide once again. Now the threefold division of the coelome into axocele, hydroseal and somatoseal is achieved.

Viewed from the left, one can see the hydropore as a dark spot near the stomach. The hydroporic canal, which connects the left hydroseal with the seawater, ends there. Older larvae are larger and broader.

Their arms are also longer. The larva propels itself with four, here dark-pointed ciliated bands, the epaulettes. At this stage, the development of the sea urchin and lager begins. This is shown in the following diagram.

The pluteus has eight larval arms. The posterior part of the body is rounded. Only two of the four epaulettes are visible. About 14 days after fertilization, the threefold division into axocele, hydroseal and somatoseal is finished.

Further development will mainly be in the left hydroseal, which is now barreled out by a large coelomic cavity. Near the left hydroseal, the epithelium of the body surface will be indented.

It thickens and lies together with the central part of the hydroseal. The outer epithelium of the hydroseal also thickens. During this process, part of the somatoseal pushes slowly between the stomach and the hydroseal.

The hydroseal is compressed by the thick ectodermal indentation. The ectodermal indentation deepens. The hydroseal changes into a peripheral ring canal,

the anaga of the ambulacral system. The edges of the indented surface of the body close together. Beneath is the vestibular cavity. The rounded coelom cavity of the hydroseal has been expanded in five places.

Here, the five primary podia develop. In the optical section, two of them are pictured. They are the first visible organs of the five-pointed, radially symmetrical ambulacral system.

There are folds of ectoderm, the epineural folds, which run parallel to the radial water canal. With the aid of the somatoseal, here dark red, the antlagon of the five teeth and the early stages of the hemal system are formed.

In this way, the epineural folds arch outwards to form the anlaga of the ectoneurological nervous system, shown here in yellow. The edges of the epineural folds grow together. The ectoneurological nerve ring develops. On the left side of the pluteus, organ anlagon are formed by indentation

or bulging of ectodermal or mesodermal material. From these, the essential parts of the oral side of the sea urchin later develop. On the right side, which later becomes the apaoral side of the sea urchin, pedicillarii have been formed on the surface of the body.

Some stages of differentiation, which lead to the radially symmetrical oral part of the sea urchin, are visible in the living pluteus. This 14-day-old pluteus has a very large left hydrosil with a thick outer coelomic epithelium.

The left coelom is rounded off. The surface of the body is indented and at the point of contact, the epithelium has thickened.

The somatosil extends and passes under the hydrosil. The robust epithelium of the ectoderm presses the hydrosil together. The radial water canal develops.

The body surface of the larva closes over the vestibule. This gives rise to the embryonic covering or amnion. The further course of development

can be seen better in a lateral view of the larva. We are now looking at the outer surface of the vestibule and can recognise bulges of the hydrosil, which are the anlagon of the ambulacral system.

The bulges rounding off at the ends of the five radial canals are the anlagon of the primary podia. In the centre, the ring canal of the ambulacral system has formed.

The five primary podia have developed towards the centre and now cover nearly the whole oral side of the sea urchin. Under stronger magnification, you can see the first calcareous plates

of the developing sea urchin. The sea urchin disk enlarges and the number of calcareous plates increases.

About 30 days after fertilisation, the sea urchin disk has reached its greatest diameter. Water currents are observable in the ring canal of the ambulacral system. They are responsible for the first movements.

During differentiation of the sea urchin oral side to the left of the larval stomach, pedicellari are formed on the surface of the body to the right. Viewed from above, each pedicellaria is seen to have three valves.

With older plutei, spines are also visible beside the pedicellari. About five weeks after fertilisation,

the Pluteus of Sammichinus miliaris is ready for metamorphosis.