Vezhdovsky to exist in the embryo of certain forms. The blood in the Chaetopoda consists of a plasma in which float a few corpuscles.
The animals thus associated, the Rotifera, Chaetopoda and Arthropoda, are composed of a larger or smaller number of hollow rings, each ring possessing typically a pair of hollow lateral appendages, moved by intrinsic muscles and penetrated by blood-spaces.
They are divided as follows: (i) Haplodrili or Archiannelida; (2) Chaetopoda; (3) Myzostomida, probably degenerate Polychaeta; (4) Hirudinea (see CHAETOPODA and LEECH); (5) Echiuroidea.
It has been insisted, by those who accepted Lankester's original doctrine of the direct or genetic affinity of the Chaetopoda and Arthropoda, that Apus and Branchipus really come very near to the ancestral forms which connected those two great branches of Appendiculate (Parapodiate) animals.
It has been indeed largely upon the conditions characterizing the Chaetopoda that the conception of the coelom in the Coelomocoela has been based.
The distinctive characters of the class Chaetopoda as a whole are partly embodied in the name.
The Chaetopoda are characterized by the possession of horny epidermic chaetae embedded in the integument and moved by muscles.
Among the simpler Chaetopoda the coelom retains the character of a series of paired chambers, showing the above relations to the exterior and to the gonads.
ARTHROPODA, a name, denoting the possession by certain animals of jointed limbs, now applied to one of the three sub-phyla into which one of the great phyla (or primary branches) of coelomocoelous animals - the Appendiculata - is divided; the other two being respectively the Chaetopoda and the Rotifera.
The nephridia of the Polychaeta have been generally dealt with above in considering the nephridial system of the Chaetopoda as a whole.