[Illustration: Fig. 128. Young Cuvieria, much enlarged; _l_ body, _g_ tentacles.]
[Illustration: Fig. 129. Somewhat older Cuvieria; _l_ body, _g_ tentacle round mouth, _g'_ testaete of sole, _b_ madreporic tentacle.]
The highest of our Holothurians in structure, is the Pentacta. (Fig. 130) It is very rare on our beaches, though occasionally found under stones at low-water mark; farther north, in Maine, and at Grand Manan, it is very common, covering all the rocks near low-water mark. It is a chocolate brown in color, and measures, when fully expanded, some fifteen to eighteen inches in length. Unlike the Cuvieria, the ambulacral suckers are evenly distributed and almost equally developed on all the tubes; between the five rows of ambulacral suckers are scattered irregularly certain appendages resembling suckers, but found on examination not to be true locomotive suckers, and called on that account false ambulacra. These are the organs corresponding to the warts around the mouth of the Synapta. Although the ambulacral suckers are, as we have said, equally developed on all the tubes, yet the Pentacta does not use them indiscriminately as locomotive organs. In Pentacta, as well as in all Holothurians, whether provided with ambulacral suckers, or, like the Synapta and Caudina, deprived of them, the odd ambulacrum, viz. the one placed opposite the madreporic body, is always used to creep upon, and forms the under surface of the animal.
[Illustration: Fig. 130. Pentacta frondosa; expanded about one third the natural size.]
The correspondence between the different phases of growth in the young Pentacta, and the adult forms of the orders described above, the Synapta, Caudina, Cuvieria, and Pentacta itself, is a striking instance of the way in which embryonic forms illustrate the relative standing of adult animals. In the earlier stages of its development, the ambulacral tubes alone are developed in the Pentacta; in this condition it recalls the lower orders of Holothurians, as the Synapta and Caudina; then a sole is formed by the greater development of three of the ambulacra, and in this state it reminds us of the next in order, the Cuvieria, while it is only in assuming its adult form that the Pentacta develops its other ambulacra, with their many suckers.