Fig. 254.—Holothuria nigra. t, Buccal tentacle or "feeler." × ½.
There are about 520 species of living Holothuroidea, and of these about twenty-one have been recorded from British waters. One of the best-known of the British species is Holothuria nigra (Fig. 254), commonly known as the "Cotton-spinner"; and this we shall take as a type for special description. The animal may attain a length of a foot when fully extended, and has a diameter of from 3 to 4 inches. It is of a very dark brown colour on one side, which in crawling it keeps uppermost, whilst on the lower side it is of a tawny yellow hue. Three of the radii (often termed the "trivium") are situated on the lower surface; two (termed the "bivium") on the upper surface. The podia are scattered fairly evenly over the whole surface without reference to the radii; below they are regular tube-feet provided with suckers, whilst on the upper surface they are pointed tentacles, employed only for sensory purposes.
If the animal be observed alive and in its natural surroundings, a ring of twenty large tentacles can be seen surrounding the mouth. These buccal tentacles are in every respect comparable with the buccal tube-feet of Ophiuroidea and Spatangoidea, and, like them, are employed in shovelling the muddy substratum on which the animal lies into the mouth.
Ludwig employs the term "feeler" for these buccal tentacles, in order to distinguish them from the pointed podia scattered over the bivium. This procedure will be adopted here. In the Cotton-spinner the feelers, when extended, show a short smooth stem, from the apex of which springs a circle of short branches, which are in turn beset with a double row of branchlets, themselves branched. Such feelers are said to be shield-shaped.
A transverse section through the radius of a Sea-cucumber is, in general, like one through the radius of a Sea-urchin; the points of difference to be noted are: (a) In the Sea-cucumber, beneath the ectoderm, is a thick dermis with small plates scattered in it, instead of the whole dermis being calcified, as is the case in the Sea-urchin; (b) the ampulla of each podium is connected with the peripheral portion by one canal, not two, as in the case of the Sea-urchin; (c) there is a development of coelomic nervous tissue from the outer side of the perihaemal canal; (d) internal to the radial water-vascular canal are to be seen cross-sections of two great bands of longitudinal muscles, by the contraction of which the body is shortened. Lengthening is brought about by the contraction of transverse muscles, which are found on the inner side of the body-wall in each interradius; the five sets taken together act like circular muscles, or a rubber band, on the incompressible fluid in the body-cavity.
When the Sea-cucumber is opened by a cut along the left dorsal interradius, the spacious coelom is laid open, and lying in it is seen the alimentary canal. This tube is bent on itself, so that it has a form like ∽ (Fig. 255, B) running backwards to the posterior end of the body, then running forwards to near the anterior end, before it finally turns to run backwards to the anus. By taking cross-sections of the body at different levels, it can be shown that the alimentary canal makes a half-turn round the longitudinal axis (Fig. 255, A). It is suspended by bands of membrane, termed "mesenteries," to the body-wall, and of these there are three, the first of which (i.e. the one nearest the mouth) is attached to the mid-dorsal interradius (Fig. 255, A, M1), the next to the left dorsal interradius (M2), and the last to the right ventral interradius (M3).
The alimentary tube shows four regions, which are distinguished as follows:—(1) A short oesophagus with strongly-marked longitudinal folds in its walls; this is separated by a constriction from (2) the stomach, a very short region, characterised by its strong musculature. Next follows (3) the intestine, a thin-walled tube comprising the middle limb and most of the descending and ascending limbs. This finally passes into (4) the wide terminal "rectum," or "cloaca," which is connected to the body-wall by muscular bands which traverse the coelom (Fig. 256, 10).
The cells lining the oesophagus resemble ectodermal cells; those lining the stomach are nearly all gland-cells, and obviously secrete the digestive juice. The powerful muscles of this portion of the gut produce a strong peristalsis which thoroughly mixes the juice with the food, and in the thin-walled intestine absorption of the digested material takes place. The extreme thinness of the intestinal wall is common to many animals (e.g. Sipunculus, Vol. II. p. [412]) which swallow mud and sand for the sake of the organic matter which they contain.
Fig. 255.—A, diagrammatic cross-section of a Holothurian; B, diagrammatic longitudinal section. I-V, radii; Int.1-Int.3, the three limbs of the alimentary canal; M1-M3, the three mesenteries attaching the same to the body-wall. (After Ludwig.)