The Balanoglossidæ include the genera Glossobalanus, Balanoglossus, and Ptychodera, the latter the oldest known member of the group, its type, Ptychodera flava, having been described by Eschscholtz from the Pacific Coast in 1825, while Balanoglossus clavigerus was found by Della Chiaje in 1829.
Low Organization of Harrimaniidæ.—Apparently the Harrimaniidæ, with simpler structure, more extensive notochord, and direct development, should be placed at the bottom as the most primitive of the Enteropneustan series. Dr. Willey, however, regards its characters as due to degeneration, and considers the more elaborate Balanoglossidæ as nearest the primitive type. The case in this view would have something in common with that of the Larvacea, which seems to be the primitive Tunicates, but which may have been produced by the degeneration of more complex forms.
[CHAPTER XXVI]
THE TUNICATES, OR ASCIDIANS
Structure of Tunicates.—One of the most singular groups of animals is that known as Ascidians, or Tunicates. It is one of the most clearly marked yet most heterogeneous of all the classes of animals, and in no other are the phenomena of degeneration so clearly shown.
Among them is a great variety of form and habit. Some lie buried in sand; some fasten themselves to rocks; some are imbedded in great colonies in a gelatinous matrix produced from their own bodies, and some float freely in long chains in the open sea. All agree in changing very early in their development from a free-swimming or fish-like condition to one of quiescence, remaining at rest or drifting with the current.
Says Dr. John Sterling Kingsley: "Many of the species start in life with the promise of reaching a point high in the scale, but after a while they turn around and, as one might say, pursue a downward course which results in an adult which displays but few resemblances to the other vertebrates. Indeed, so different do they seem that the fact that they belong here was not suspected until about thirty-five years ago. Before that time, ever since the days of Cuvier, they were almost universally regarded as mollusks, and many facts were adduced to show that they belonged near the acephals (clams, oysters, etc.). In the later years when the facts of development began to be known, this association was looked on with suspicion, and by some they were placed for a short time among the worms. Any one who has watched the phases of their development cannot help believing that they belong here, the lowest of the vertebrate series."