Shell thin; operculum absent; tentacles bifid; foot secretes a float; pelagic. Janthina.
The upper layer is the corneagen and secretes the lens, the lower is the retinal layer.
The ectoderm loses entirely the ciliation which it had in the planula and actinula stages and commonly secretes on its external surface a protective or supporting investment, the perisarc. Contrasting with this, the anthopolyp is generally of s q uat form, the diameter often exceeding the height; the peristome is wide, a hypostome is lacking, and the ectoderm, or so much of it as is exposed, i.e.
In many Lamellibranchs a gland is found on the hinder surface of the foot in the mid line, which secretes a substance which sets into the form of threads - the so-called " byssus " - by means of which the animal can fix itself.
In Aspergillum the whole of the tubular mantle area secretes a continuous shelly tube, although in the young condition two valves were present.