Spination

The internal defensive spicula are very numerous, and appear all to spring from the basal membrane, their diameter is nearly the same at the base, but they vary greatly in their length and also in the degree of their spinations; in some the spines are quite in an incipient state, while in others they are well and distinctly produced.

They share few characters: 25 vertebrae; compact, triangular gut; inconspicuous gas bladder; and some head spination.