Fig. 257.—Types of calcareous rings and of ossicles. A, calcareous ring of Phyllophorus rugosus, × 2; B, calcareous ring of Holothuria cinerascens; C, ossicle of Holothuria atra; D, ossicle of Holothuria fusco-rubra. r, Radial piece. (After Ludwig.)
The genital organs consist of a single group of branched tubes situated on the left side of the dorsal mesentery, which converge to open into a short genital duct, which leads to a pore situated in the mid-dorsal line, a short distance behind the feelers. From the common point of origin of the tubes, the "genital base," as it is called, a worm-shaped genital stolon[[507]] extends back along the genital duct towards the body-wall. There is no genital rachis.
Classification of Holothuroidea.
The class is in many points of structure exceedingly variable, but many striking variations in important organs occur in allied species, and even in the same species, and hence are probably not of physiological importance. We shall therefore confine our attention mainly to those differences in structure which are correlated with differences in habits, and therefore of systematic importance. We shall consider in order (1) the feelers; (2) the method of protecting these; (3) the rest of the water-vascular system; (4) the gills; and (5) the skeleton.
Feelers.—These organs have been made the basis of the division of the Holothuroidea into orders, and as they are the means by which food is obtained, and are thus of first-class physiological importance, this procedure is fully justified. In three orders they have the shield-shaped ends described in the case of Holothuria nigra, but in another large order (Dendrochirota) they are much branched, and end in a mass of delicate twigs. In another order (Synaptida) they are feather-shaped, with two rows only of branches, whilst finally in Molpadiida they are simple finger-shaped processes with one or two lateral branches. The number of the feelers varies from ten to thirty.
In the Dendrochirota the entire anterior portion of the body can be introverted into the interior, so that in this way the crown of feelers can be effectively protected. The retractor muscles are modified portions of the longitudinal muscles of the body-wall, which traverse the body-cavity, and are inserted into the radial pieces of the calcareous ring. Similar muscles are found in the genus Molpadia and in many of the Synaptida. In Aspidochirota and Pelagothuria they are totally wanting, and here the feelers possess long ampullae which allow of the tentacles being individually contracted to very small dimensions. These ampullae seem to be present in nearly all cases in Molpadiida, and in Synaptida, although in the last-named order they are very feebly developed, and must be looked on as vestigial. In Dendrochirota, owing to the strongly developed retractors, they would be useless, and so are absent.
Water-Vascular System.—In Synaptida the radial canals are totally absent in the adult, and the only podia are the feelers, which spring directly from the ring-canal. The radial canals are present in Pelagothuria, but the feelers are still the only podia; in the Molpadiida there are only five small terminal tentacles round the anus in addition to the feelers. In the Elasipoda all the podia have pointed ends, but the dorsal podia are few, long, and stiff, and often coalescent in places to form grotesque or remarkable appendages. In the remaining forms the podia of the trivium have always suckers, whilst those of the bivium may or may not be pointed. In Psolus the two dorsal radial canals and their podia are totally absent.
Respiratory Trees.—These are present in Aspidochirota, Dendrochirota, and Molpadiida, totally absent in the Synaptida and Pelagothuria, and doubtfully represented in a few Elasipoda by a single unbranched outgrowth of the gut.
Skeleton.—This consists, as explained above, of the scattered deposits in the skin and of the calcareous ring. As regards the first, their shape varies immensely, and yet one or two principal types characteristic of each of the main divisions can be defined. Thus the Synaptida are characterised by wheels, with spokes ending in a hub, and by anchors attached to a plate. The Elasipoda have simple St. Andrew's crosses, whilst the Aspidochirota are mainly characterised by "stools" (Fig. 257, C) and buckles (Fig. 257, D). The Dendrochirota have a bewildering variety of forms; the most characteristic, however, are a right-angled cross and a grating, very similar to the buckles of the Aspidochirota, except that in the former there are usually four holes placed cross-wise, whilst the buckle has generally two parallel rows of three holes. Since these ossicles are the only records we possess of the existence of fossil Holothuroidea, they have been studied with great care. The calcareous ring varies very much. The radials are always five (except in individuals where there are more than five radii), but the interradials are increased in the Synaptida, and in the other orders are in some cases diminished or occasionally suppressed altogether. The last is the case in nearly all Elasipoda; here the radials consist of a central horizontal piece with two diverging arms at each side. These arms, which can branch repeatedly, traverse the adjacent interradii, meeting those of the next radii, so that interradials are in most cases entirely absent. The Aspidochirota have usually a ring consisting of small squarish ossicles (Fig. 257, B). In the Molpadiida and Dendrochirota the radials are prolonged backwards into forked tails, which in some Dendrochirota are broken into a number of small pieces (Fig. 257, A), the lower parts of the interradialia being similarly divided.
The classification of the Holothuroidea is comparatively easy. All authors recognise six divisions, and the only dispute is as to whether they are to be regarded as families or orders. Ludwig[[508]] divides the group into two orders, Paractinopoda and Actinopoda, but the first includes only those forms which have lost the radial canals, and this is only one step farther in a degeneration, intermediate stages of which can be traced in the other divisions. There is really no ground for placing the Paractinopoda in contrast to all the other divisions, and the only alternative is to regard the six main divisions as orders, since a class must be divided into orders. In the case of only one, however, is a further division into families practicable, and therefore each of the others will contain a single family.