It has been shown by C. Janet (1889), from careful studies of the musculature, that the greater part of the head-capsule is built up of the four anterior head-segments, the hindmost of which has the mandibles for its appendages, and this conclusion is in the main supported by the recent work on the head skeleton of J.
Lacuna, foot with two posterior appendages, marine, entirely aquatic. Cremnoconchus, entirely aerial, Indian.
The appendages of the body are reduced to branchiae, present in certain forms. A clitellum is present.
The ersaeome (Calycophorida), made up of the same appendages as the preceding type but with the addition of a nectocalyx; when free termed Ersaea.
The neuromeres are shown in Arabic, the appendages in Roman numerals.