The velum is also provided with a circlet of twelve tantacles (in some species sixteen) which hang backwards into the pharynx; these are the velar tentacles.
Head broad, surrounded by a funnel-shaped velum or hood; no radula; dorsal appendages foliaceous.
The circular system is developed continuously over the entire subumbral surface, and the velum represents a special local development of this system, at a region where it is able to act at the greatest mechanical advantage in producing the contractions of the umbrella by which the animal progresses.
The sub-umbrella invariably shows a velum as an inwardly projecting ridge or rim at its margin, within the circle .of tentacles; hence the medusae of this sub-class are termed craspedote.
Under ordinary circumstances the horse breathes entirely by the nasal passages, the communication between the larynx and the mouth being closed by the velum palati.
Correlated with the well-developed muscular system and sense-organs of the medusa, we find also a distinct nervous system, either, when there is no velum, in the form of concentrations of nervous matter in the vicinity of each sense-organ, or, when a velum is present, as two continuous rings running round the margin of the umbrella, one external to the velum (exumbral nerve-ring, n.r l, see fig.
Later the ciliated ring or velum disappears and seven imbricated calcareous plates, made up of flattened spicules, are formed on the dorsal surface.
Vr, Velum or circlet of ciliated cells.
The palate is long and narrow; its mucous surface has seventeen pairs of not very sharply defined oblique ridges, extending as far back as the last molar tooth, beyond which the velum palati extends for about 3 in., having a soft corrugated surface, and ending posteriorly in an arched border without a uvula.
The foot arises as a prominence on the ventral surface and grows forward, and at the end of five or six days the velum atrophies and the foot becomes the organ of locomotion; the animal then ceases to swim and sinks to the bottom.