The whole of this muscular case is lined with a delicate membrane or peritoneum, from which processes pass inwards to support the various viscera.

(532). But although the calcareous shell of the Echinus is thus totally lost, the locomotive suckers or feet already described are still the principal agents employed in progression. In many species, as in that represented in the annexed figure (fig. 100), these organs are distributed over the whole surface of the animal, and are protruded through countless minute orifices that perforate the integument. In other cases, as in H. frondosa, they are arranged in five series, resembling the ambulacra of an Echinus; and in some instances they are only found upon the middle of the ventral surface of the body, that forms a flattened disk upon which the animal creeps, somewhat in the manner of a snail.

Holothuria.

Fig. 100. Holothuria.

The ambulacral feet themselves, represented on an enlarged scale at c, precisely resemble in all the details of their structure those of the Asterias, and their protrusion and retraction are effected in the same manner; but, in addition to these organs, we find in some genera moveable hooks or spines (fig. 100, d), which are likewise retractile, and most probably assist in locomotion.

(533). The mouth is a round aperture, as wide as the bore of a goose-quill, placed in the centre of a raised ring at the anterior extremity of the body (fig. 100, a.) Around the oral orifice is placed a circle of ten-tacula, which are apparently extremely sensitive, and serve perhaps not only as instruments of touch, but as prehensile organs, used for the capture of prey, or for assisting in deglutition. "When the sphincter muscle that closes the mouth contracts, the tentacles are withdrawn, and become no longer visible externally; in this state, on opening the animal (fig. 101, b), they are found to resemble long caeca appended to the commencement of the oesophagus, and have been described by some authors as forming a salivary apparatus.

(534). The total deficiency of an external skeleton or calcareous framework precludes, of course, the possibility of the existence of any complex dental apparatus resembling the "lantern of Aristotle;" the only vestige of the complex teeth of the Echinidae which here remains is a small circle of calcareous pieces, surrounding the opening of the mouth. These plates, from their extreme friability, have been aptly enough likened to laminae of dried paste: they may indeed, in some slight degree, be efficient in bruising food taken into the mouth; but it is more probable that they merely form points of insertion for the longitudinal muscles of the body, which, thus fixed around the circumference of the oral orifice, will by their contraction powerfully dilate that aperture for the purpose of taking in nourishment.

(535). The alimentary canal is of great length, but, like that of the Echinus, presents no stomachal dilatation; from the mouth (fig. 101, a), in which a bristle is placed, it descends to the anal extremity of the body, where, turning upon itself, it again mounts up towards its commencement, whence turning back again, and forming numerous convolutions (d d d), it once more passes backwards, and becoming constricted near its termination, opens into a large membranous cavity (e) that may be called the cloaca. Throughout the whole of this long course, the alimentary tube is surrounded with a membrane derived from the peritoneal lining of the visceral cavity, which forms delicate mesenteric folds connecting it to the walls of the body and supporting it through its entire length. The whole intestine is generally found distended with sand, wherein may be detected the debris of corals, algae, fuci, and other marine substances.

(536). In the structure of the respiratory apparatus, the Holothuridae differ materially from the rest of the Echinodermata, and, in fact, from all other animals. In the Holothuria, the aeration of the circulating fluid is provided for by allowing the surrounding element freely to enter into the internal parts of the creature; but instead of bathing the surfaces of the viscera, the water is confined in a peculiar system of ramifying canals, forming a structure of great beauty and, from its singularity, extremely interesting in a physiological point of view. We have seen that the intestinal canal terminates in a membranous receptacle or cloaca (fig. 101, e), contained within the cavity of the abdomen, to the walls whereof it is attached by delicate fleshy bands: this cloacal cavity communicates with the exterior of the body by a wide orifice twice as large as the aperture of the mouth, through which, in the figure, a bristle (f) has been passed; and it is by this orifice that the water required for the purpose of respiration is taken in, and then forced, by the muscular walls of the cloaca itself, through the whole system of respiratory canals whereby its distribution is effected. The organs of respiration commence at the upper part of the cloaca, near the termination of the intestine, by a large opening leading to a wide membranous tube, which immediately divides into two vessels (g g) forming the main trunks of the beautiful arborescent branchiae; these extend to the opposite extremity of the body, giving off in their course numerous lateral branches that divide and subdivide, so as to form what has been not inaptly termed the "respiratory tree," until they ultimately terminate in minute vesicular caeca, into which the water derived from the cloaca of course penetrates.

One division of this elegant apparatus is maintained in close contact with the walls of the body by a series of delicate tendinous bands, while the other becomes applied to the convolutions of the intestines, wherewith it is likewise united. It is this last-mentioned division that would appear to be specially provided for the oxy-genization of the nutritive fluids.