Acraspeda

Acraspeda, a sub-class of Hydrozoa, including the majority of the large permanently unattached jelly-fish. The main characteristic, from which the name is derived, is the absence of a velum. The body consists of a bell-shaped disc, in which the polypite is suspended; the structure may be compared to an open umbrella, with a very short handle. In the craspedote jellyfish a velum or shelf runs round the umbrella, a little above the base, and limiting the opening; this is absent in the Acraspeda. The most interesting feature in this group is its development. The life history is divided into three stages, excluding the embryonic. After the free-swimming ovum has become fixed, it develops into a small hydra-like body, the Scyphistoma (this stage is not known in many forms). By a series of constrictions this tube becomes transversely divided, and then resembles a pile of saucers with ragged edges; this is the Strobila stage. The constrictions deepen and successive segments are cut off; these swim away as Ephyrae, each of which develops into the adult form, which is sometimes of a gigantic size. The four bodies on the margin of the disc, which serve as sensory organs, are covered by hoods. The group is, therefore, often known as the "covered-eyed Medusae." Aurelia, one of the commonest of the larger jelly-fish round the British coasts, serves as a good type of the class.