The species included are not, so far as is known, haemal parasites.
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.
- Flagellates, in the great majority of instances haemal parasites, derived from a biflagellate, Bodo-like type, in which the posteriorly-directed (trailing) flagellum is always present and attached to the body by an undulating membrane, of which it constitutes the thickened edge.
Probably this is not a haemal parasite, and lacks an alternate host.
Towards the end of the body both neural and haemal arches are continued into forked radial cartilages, which support a median fin.
All living Echinoderms have a lacunar, haemal system of diverse origin; this, the ambulacral system, and the coelomic cavities, contain a fluid holding albumen in solution and carrying numerous amoebocytes, which are developed in special lymph-glands and are capable of wandering through all tissues.