Thus in the Chaetopoda the perivisceral cavity is coelomic; in this respect the group contrasts with the Arthropoda and Molluscs, where the perivisceral cavity is, mainly at least, part of the vascular or haemal system, and agrees with the Vertebrata.
This is the vascular or haemal system (formerly and unnecessarily termed pseudhaemal).
This state of affairs has no antecedent improbability about it, since in the Vertebrata the coelom is unquestionably confluent with the haemal system through the lymphatic vessels.
Fins without spines; caudal fin, if present, without expanded hypural, perfectly symmetrical, and supported by the neural and haemal spines of the posterior vertebrae, and by basal bones similar to those supporting the dorsal and anal rays.
Trypanosomes were next seen in human blood 1 Trypanophis, although lacking (so far as is known) a haemal habitat, is included here, since it is undoubtedly closely related to Trypanoplasma.