They exist in the Brachiopoda (which are probably not unrelated to the Chaetopoda), but otherwise are absolutely distinctive of the Chaetopods.
BRACHIOPODA, an important and well-defined but extremely isolated class of invertebrates.
In a later scheme based on our increased knowledge of fossil forms, the Brachiopoda are divided into four primary groups (orders).
22) shows that it consists of a stout base, composed of a very hyaline connective tissue not uncommon in the tissues of the Brachiopoda, which is traversed by certain canals whose nature is considered below under the section (The Body Cavity) devoted to the coelom.
The number and position of the muscles differ materially in the two great divisions into which the Brachiopoda have been grouped, and to some extent also in the different genera of which each division is composed.