Tornaria (fig. 229) cannot be definitely united either with the Trochosphere or with the echinoderm larval type.
It was at first thought that we had before us an echinoderm in the act of transformation.
This larval form, which is shewn in fig. 264 A, is the type from which the various forms of echinoderm larv start.
Actinotrocha (fig. 230) undoubtedly resembles more closely echinoderm larv than the Trochosphere.
A resemblance to the other echinoderm larvae is found in the fact that coelomic diverticula of the enteron are present.
It also agrees with the echinoderm larv in the absence of sense organs on the proral lobe.
In the body of an echinoderm can be usually recognized an upper or dorsal surface and a lower or ventral surface.
The tube-feet usually have a tiny sucking disk at the tip, and by means of them the echinoderm can cling very firmly to rocks.
How far Actinotrocha is related to the echinoderm larv cannot be settled.
Half a dozen varieties of these are known, one of which inhabits an echinoderm, another a decapod crustacean.
Any of various marine invertebrates of the phylum Echinodermata, having a latticelike internal skeleton composed of calcite and usually a hard, spiny outer covering. The body plans of adult echinoderms show radial symmetry, typically in the pattern of a five-pointed star, while the larvae show bilateral symmetry. Echinoderms probably share a common ancestor with the hemichordates and chordates, and were already quite diversified by the Cambrian Era. They include the starfish, sea urchins, sand dollars, holothurians (sea cucumbers), and crinoids, as well as thousands of extinct forms.