Fig. 253.—Hyboclypus gibberulus. × 1. a, Anus. (After Zittel.)
Turning now to the Mesozoic forms with an excentric anus, there were a number of forms which have been grouped together as Holectypoidea which had auricles and teeth and gills, although these were only feebly developed, and in which the pore-plates remained separate. The periproct was a comparatively large area, and in Pygaster, as in the surviving form Pygastrides, it was in contact with the apical system, although outside it. Many of the genera were of considerable height in proportion to their length. In Conoclypeus and Discoidea the jaws and auricles were very weak. The Echinoconidae have only vestigial auricles, and on this account are often definitely grouped with the Spatangoidea, but they are closely allied to the Holectypoidea. They are Cretaceous forms of high conical shape (Galerites). In Hyboclypus (Fig. 253) all trace of the teeth has disappeared, but the periproct is large and in contact with the apical system; these forms appeared in the Jurassic. The Collyritidae, also Jurassic, had a marginal anus. The apical system was so much elongated that two of the ocular plates are widely separated from the other three, two opposite interambulacra meeting between them. Unmistakable Spatangoidea (Spatangidae and Ananchytidae) appear in the Cretaceous, true Clypeastroidea (Fibularites) in the Cretaceous, the other families in the Tertiary.
Reviewing these facts, we see that from the Holectypoidea we can pass by insensible steps on the one hand into true Clypeastroidea, and on the other hand into true Spatangoidea. The Holectypoidea differed from Endocyclica only in the position of the anus, and the initial step in the backward shift of this organ is seen in Pygaster. One result follows from this conclusion, that the modification of the dorsal tube-feet into breathing organs, and the consequent appearance of petals which accompany the taking on of burrowing habits, were independently developed in the Clypeastroidea and Spatangoidea, since these features were absent in the more primitive members of both groups.
CHAPTER XIX
ECHINODERMATA (CONTINUED): HOLOTHUROIDEA = SEA-CUCUMBERS
CLASS IV. HOLOTHUROIDEA
This class of the Eleutherozoa comprises those sausage-shaped, leathery Echinodermata familiarly known as Sea-cucumbers. They are named Holothuroidea from ὁλοθούριον, an animal described by Aristotle, and believed to belong to this class.
The Holothuroidea resemble Echinoidea in the fact that the radial canals of the water-vascular system run backwards and upwards from the ring-canal over the surface of the body, terminating in small papillae near the anus, which, as in the Echinoidea Endocyclica, is situated at the upper pole of the body. There are, of course, no arms; and a further resemblance to Echinoidea is shown by the fact that the ambulacral grooves are represented by closed epineural canals, and that the ectoderm consists of long, slender, flagellated cells interspersed with gland-cells, underneath which is a plexus consisting of nerve-fibres and small bi-polar ganglion cells. There are, however, no spines or pedicellariae; and Holothuroidea differ not only from Echinoidea, but from all other Echinodermata, in the vestigial character of their skeleton, which consists merely of isolated nodules of calcium carbonate embedded in the skin. The body-wall is provided with transverse muscles running across the interradii, and also with powerful longitudinal muscles, running along the radii, by means of which worm-like contractions are carried out. Similar muscles, though much less developed, occur in the Echinothuriidae, and must have been present in many extinct Echinoidea in which the plates of the corona overlapped; and hence it is exceedingly probable that from some of these early forms, as, for instance, Bothriocidaris, Holothuroidea may have been evolved. The muscular body-wall has indeed been as important a factor in the evolution and differentiation of the Holothuroidea as the muscular arm in that of Ophiuroidea, or the movable spine in the case of Echinoidea.
