This is one of the most curious of the Holothurians, and easily observed on account of its transparency, which allows us to see its internal structure. It has a long cylindrical body ([Fig. 124]) along the length of which run the five rows of ambulacra, which are in this instance closed tubes without any projecting suckers or locomotive organs of any kind attached to them, so that the name is retained only on account of their correspondence in position, and not from any similarity of function to the ambulacra in Star-fishes and Sea-urchins. But though the ambulacra in Synapta are in fact mere water-tubes like the vertical tubes in the Ctenophoræ, by means of which the water, first filtered through the madreporic body, circulates along the skin, they are as organs perfectly homologous with the ambulacra in all other Echinoderms. The mouth has a circular tube around the aperture, and a wreath of branching tentacles encircling it. The habits of these animals are singular. They live in very coarse mud, but they surround themselves with a thin envelope of finer sand, which they form by selecting the small particles with their tentacles, and making a ring around their anterior extremity. This ring they then push down along the length of the body, and continue this process, adding ring after ring, till they have entirely encircled themselves with a sand tube. They move the rings down partly by means of contractions of the body, but also by the aid of innumerable appendages over the whole surface. To the naked eye these appendages appear like little specks on the skin; but under the microscope they are seen to be warts projecting from the surface, each one containing a little anchor with the arms turned upward ([Fig. 125]). Around the mouth [fig 125]
Fig. 125. Anchor of Synapta; a anchor,
w plate upon which anchor is
attached; greatly magnified. these warts are larger, but do not contain any anchors. It will be seen hereafter that these appendages are homologous with certain organs in other Holothurians, the warts with the anchors corresponding to the limestone pavement covering or partially covering the surface of the Cuvieria, for instance, while those without anchors correspond to the so-called false ambulacra in Pentacta. By means of these appendages, though aided also by the contractions of the body, the Synaptæ move through the mud and collect around themselves the sand tube in which they are encased. Their food is very coarse for animals so delicate in structure. When completely empty of food they are white, perfectly transparent, and the spiral tube forming the digestive cavity may be seen wound up and hanging loosely in the centre for the whole length of the body. In such a condition it is of a pale yellow color. But look at one that is gorged with food. The whole length of the alimentary canal is then crowded with sand, pebbles, and shells, distinctly seen through the transparent skin, and giving a dark gray color to the whole body. They swallow the sand for the sake of the nutritious substance it contains, and having assimilated and digested this, they then eject the harder materials. The motion of the body in consequence of its contractions is much like that of leeches, and on this account these Synaptæ were long supposed to be a transition type between the Radiates and worms. The body grows to a great length, often half a yard and more, but constantly drops large portions from its posterior part, by means of its own contractions, or breaks itself up by the expulsion of the intestines, which are very readily cast out. The tentacles are hollow, consisting of a central rib with branches from either side. In the Synaptæ, as in all the Holothurians, the madreporic body is placed near the mouth, between two of the ambulacra, and opposite the fifth or odd one. The tube, connecting with the central tube around the mouth, by means of which it communicates with the ambulacral tubes, is very short.
Caudina. (Caudina arenata Stimps.)
Fig. 126. Caudina arenata; natural size.
Several other Holothurians are frequently met with on our shores. Among them is the Caudina arenata ([Fig. 126]), a small Holothurian, yellowish in color, and thick in texture, by no means so pretty as the white transparent Synapta; the tentacles are short, resembling a crown of cloves around the mouth. It lives in the sand, and may be found in great numbers on the sandy beaches after a storm.
Cuvieria. (Cuvieria squamata D. & K.)