In these the tentacles are stunted or suppressed and the mesenteries are ill-developed, but the sulcus is unusually large and has long cilia.
These grooves are known as the sulcus and sulculus, and will be more particularly described hereafter.
There is only one ciliated groove, the sulcus, in the stomodaeum.
The stomodaeum is compressed laterally, and is furnished with two longitudinal grooves, a sulcus and a sulculus.
The third pair is formed in the sulcar chamber, in close connexion with the sulcus, and in this case the muscle-banners are on the sulcular faces.