Sea Urchin Dissection



Sea Urchin Dissection

The echinoids, or urchins, resemble other echinoderms in their general features. They differ in that the dermal ossicles are thin plates fused rigidly together to form a hollow test that encloses most of the animal. The test is covered by an abundance of movable spines. Modified tube feet are the major respiratory organs and the madreporite is aboral.

The class is divided into regular and irregular urchins. The former are the sea urchins with radial symmetry, nearly spherical bodies, and long spines. The irregular urchins are the sand dollars, sea biscuits, and keyhole, heart, and cake urchins. The body is usually flattened and the spines short.

Regular urchins possess a feeding apparatus known as Aristotle's lantern used for scraping food from hard substrata. The lantern is reduced in irregular urchins. 

External Anatomy

General

Strongylocentrotus, like most regular urchins, has the shape of a sphere with one flattened pole. The lower, oral pole, is flattened whereas the opposite, aboral pole, is rounded. The animal is pentamerously radially symmetrical. Five ambulacral axes extend around the surface of the sphere from oral to aboral poles.

The surface of the body is firm and rigid due to the underlying test of fused calcareous ossicles. The test is hollow and most of the animal's soft parts are inside it. The test is an endoskeleton, located in the dermis of the body wall and covered by a thin, inconspicuous, ciliated epidermis. Because the epidermis is not visible, the test appears to be external.  

Spines

Most of the surface bears articulated, movable spines, which are also part of the connective tissue skeleton and are also covered by a thin epidermis. The spines of S. drobachiensis are of two sizes but are similar in shape. The basal end of each spine bears a socket that articulates with a ball, or tubercle, on the test. Tiny muscles extend from the base of the spine to the surface of the test and move the spine.

( 1a. Pull a large spine from the test. Look at the center of its base for the socket. Find its tubercle on the test.  

Peristome

The center of the oral surface is free of spines and is not underlain by skeleton, there being a large hole in the test at this position. This region is the peristome (peri= around, stome= mouth.)

The triangular white tips of the five teeth of Aristotle's lantern can be seen protruding from the mouth. The space inside the mouth opening is the pharynx.

Periproct

The periproct is a small region in the center of the aboral surface surrounding the anus (peri=around, proct=anus.) The periproct is much smaller than the peristome and, since it is surrounded by the spines of the aboral hemisphere, is harder to see. The surface of the periproct is flexible.

The periproct is at the exact center of the aboral surface but the anus is a little off center, near one side of the periproct. The anus is on a slight elevation and is surrounded by an irregular array of small spines. The elevation bears numerous tiny, spiny knobs. The anus may be hard to find.

The periproct is surrounded by five large genital ossicles, or plates. One of the genital plates is modified to form a madreporite and this one is visible externally (Fig 1). The large, triangular madreporite lies to one side of the periproct. Its surface bears obvious perforations and is covered by a ciliated epithelium. The madreporite is penetrated by numerous ciliated channels continuous with the water vascular system in the interior.

Internal Anatomy

Test

The test is composed of 20 meridional rows of flattened calcareous plates, or ossicles. These are homologous to the dermal ossicles of other echinoderms and differ from them primarily in that they are thin, flat, and fused together. They form a rigid endoskeketon whose articulations are fixed and immovable.

Note that in Strongylocentrotus, as in all regular urchins, the periproct is at the aboral pole and is the aboral center of the radiating ambulacra. Similarly, the peristome is at the oral pole and is the oral ambulacral center.  

Soft Anatomy

You should now reveal the viscera by removing the test from the aboral hemisphere. The organs should remain, as undisturbed as possible, in the cup formed by the oral hemisphere. The object is to remove the aboral hemisphere of the test while leaving the periproct intact, but unsupported, with the rectum and axial complex still attached to it as in Figure 2).

( Force the sharp point of a pair of heavy scissors through the edge of body wall along the equator. Cut around the urchin to open. Be careful not to destroy any internal parts.

Reproductive System

The large space revealed by this operation is the perivisceral coelom. The most obvious organs are the five gonads (Fig 2). The gonads are on interambulacral axes. Urchins are dioecious and the gonads are either ovaries or testes. Carefully remove the gonads without disturbing any other tissues, especially the gut lying near them.  

Digestive System

Notice the complicated, white structure in the center of the floor of the body cavity. This is Aristotle's lantern, the device that supports and operates the five teeth in the mouth (Fig 2).

The mouth opens into a spacious pharynx inside the lantern. The pharynx rises vertically through the core of the lantern and emerges from it as the esophagus (Fig 2).

Shortly after exiting the lantern, the esophagus turns toward the periphery and, upon reaching the body wall, widens dramatically to become the stomach.

The stomach makes a counterclockwise circle around the inside of the test on the floor of the perivisceral coelom. At the end of its circle it reverses direction and makes a clockwise circle aboral to the stomach.

This second circle is the intestine, which is wide and flat like the stomach but paler in color. It is attached by mesenteries relatively high on the walls of the test and consequently is often damaged during removal of the aboral hemisphere of the test. The stomach is also attached by mesenteries but lower on the test and is less likely to be damaged. The stomach and intestine are very delicate and easily torn. They are difficult to study if they have been damaged so do your best to keep them intact. The intestine is often filled with irregular, green or brown food pellets. Make a small opening in the intestine o reveal the pellets.

Upon completing its clockwise loop the intestine becomes the rectum which extends aborally to the anus in the periproct (Fig 2).

Aristotle's Lantern

( Remove the remainder of the gut, axial complex, and madreporite to expose the lantern. It is a complex of 40 skeletal elements (ossicles) and 60 muscles that support and operate five teeth. In regular urchins the teeth are used for scraping algae and other organisms from rocks.

The lantern is composed of five jaws, each composed in turn of eight ossicles, one of which is the tooth. The ossicles are held in place by muscles and connective tissue. The teeth are recognizable internally as five flexible, curved, vertical rods emerging from the aboral end of the lantern on the interambulacral axes (Fig 2). They are exceedingly hard at the cutting tip but are soft at the aboral end. The teeth are worn away at the hard oral end but new tooth material is secreted continually at the aboral end.  

Figure 1 The aboral pole of a cleaned sea urchin test. [pic]

Figure 2. Aboral view of a sea urchin from which the aboral hemisphere has been removed. A portion of the intestine has been removed to reveal the underlying stomach. The rectum and axial complex are shown attached to the periproct. Arrows show the path of food movement.

[pic]

Fig 3 The central features of the water vascular system of sea urchin [pic]

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