Orders: Gymnolaemata (Sub-orders: Tripostomata, Cryptostomata, Cyclostomata, Ctenostomata, Cheilostomata), Phylactolaemata.
Gymnolaemata (Allman).- Lophophore circular, with no epistome.
This is continuous with the general body-cavity in the Phylactolaemata, while in the Gymnolaemata it develops in the bud as a part of the body-cavity, from which it becomes completely separated.
In a few Gymnolaemata the ectocyst is merely chitinous, although in most cases the four vertical walls and the basal wall of the zooecium are calcareous.
The bodywall is greatly simplified in the Gymnolaemata, in correlation with the functional importance of the skeletal part of the wall.
In some Gymnolaemata, polypides which develop an ovary possess a flask-shaped "intertentacular organ," situated between two of the tentacles, and affording a direct passage into the introvert for the eggs or even the spermatozoa developed in the same zooecium.
In the Gymnolaemata protrusion is effected by the contraction of the parietal muscles, which pass freely across the body-cavity from one part of the body-wall to another.
The alimentary canal, which may be represented by a vestigial structure, is accordingly not functional, and the larva does not become pelagic. A pyriform organ is present in most Gymnolaemata as well as the sucker by which fixation is effected.
- For general accounts of the structure and development of the Polyzoa the reader's attention is specially directed to 12, 14, 6, 25, I, 2, 17, 26, 18, 23, 3, in the list given below; for an historical account to i; for a full bibliography of the group, to 22; for fresh-water forms, to 1-3, 7-10, 17; for an indispensable synonymic list of recent marine forms, to 15; for Entoprocta, to to, II, 24; for the classification of Gymnolaemata, to 21, 1 4, 4, 13, 20; for Palaeontology, to 27, 22.