There are Sea-Urchins that carry their precautions against sinking to an extreme degree. These are the Shield-Urchins or Clypeastridæ, so-called from their flat shape; they include the American forms popularly known as "sand-cakes." The diagram (Fig. 36) shows one of the most curious of these flattened forms adapted for moving over fine sand and ooze, and literally "as flat as a pancake." The mouth is approximately in the centre of the lower surface, _B_; the upper surface, _A_, shows a rosette pattern on the top of the shell. This is formed by the rows of holes for the very minute tube feet. In the English Sea-Urchin above described, which is one of the group called (for that reason) Regulares, the rows of holes are uniformly continued all along the rounded sides of the body down to the neighbourhood of the mouth. Here they are much restricted, forming merely a rosette at the top of the shell: hence they are described as circumscript or "petaloid." The excretory aperture is shown in the photograph as a smaller dot on one side of the mouth, while in the Echinus, on the contrary, it is at the top of the shell. The five odd-looking, elongated holes are a curious individual peculiarity of this Sea-Urchin. It has already been explained that the Shield-Urchins are flattened in order to distribute their weight; these holes are a contrivance for still further reducing the weight in comparison with the area. This is when the animal is lying quiet at the bottom of the water, but when it moves about what effect will the presence of the holes produce? Flattened animals are usually supposed to derive an advantage from the fact that they sink more slowly through depths of water; as in lying upon the ground, their weight is distributed, and they float, as it were, in the same stratum of water without sinking further down. This creature, on the contrary, has apparently feared lest it should move too slowly when it moves in a vertical direction, and it presents us with an arrangement by means of which its sinking through water is facilitated. Water will pass readily through the five holes as the animal goes either up or down, and the resistance of the whole flat area to the water is thus reduced and vertical movement rendered more easy. Thus, by one and the same contrivance, the animal has lessened its weight when lying quiet, and diminished the resistance it meets with when it moves. The distribution of the holes, moreover, is such as to regulate the animal's position in sinking, and to prevent it from falling "headlong." For although the creature has, strictly speaking, no "head," yet the end nearest the mouth is the thickest and heaviest part of the "cake," and would naturally tend downwards. This tendency is counteracted by the fact that the thicker end is unperforated, while the thinner and lighter end has a large central hole to diminish its resistance and enable it to sink more rapidly.