In the sexual organs, however, both parts separate into membranous capsules or cysts, and glandular contents, as in the roe or ovary; milt or testes; kidneys, with furthermore the oviduct, penis, and urinary cyst. The latter are sentient membranes. Such is the case in the Bivalve Mollusca, and Snails.

In the next place the tegument becomes, for the first time, a self-substantial organ of sensation by appearing as an envelope of the body; the vesical form is then repeated, and by this means the annular character originates as a series of cysts arranged one behind the other; it is thus a veritable skin, from which, finally, the members sprout forth, as in Worms and Insects.

The Dermatozoa will accordingly range in three stages.

1. Blasto- or Oozoa.
2. Sexual animals.
3. Cutaneous "

An annulated tegument, cutis or true skin, appears for the first time in the Worms, with here and there lateral filaments and tentacula. True feet and antennæ appear in the Crustacea or Crabs. Lastly, feet and wings in the Insecta or Flies.

3086. The external sexual parts, especially the male, first make their appearance, and that indeed with a very striking amount of development, in the Snails, and in like manner the body of the Bivalve Mollusca or Mussels has become almost a complete mass of ova or roe. In the Cuttle-fishes the first traces or rudiments of urinary organs appear. The animals which belong to this group are accordingly the Conchozoa or Shell-animals.

3087. Animals, which are directly resolvable into sexual fluids, or that represent parts of the ovum, are the gelatinous Infusoria, Polyps, and Acalephæ. Unto this category belong the Protozoa or Mucus-animals.

3088. The complete subdivision of animals, according to the organs of sense, would consequently stand thus:

I. DermatozoaInvertebrata.
1. OozoaProtozoa.
2. Glandular animalsConchozoa.
3. Cutaneous "Ancyliozoa.
II. GlossozoaPisces.
III. RhinozoaReptilia.
IV. OtozoaAves.
V. OphthalmozoaThricozoa.

3089. Unto these organs of sense the anatomical, or internal parts, are subordinated, and range parallel to them in a striking manner. The following is their order of succession in accordance with that of their origin: