Page:Scientific results HMS Challenger vol 18 part 1.djvu/218

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Definition.— without skeleton.

The suborder comprises all those  in which no skeleton is developed. The whole body is therefore soft—a true malacoma—and is composed only of two essential parts, the central capsule and the enveloping extracapsulum. The suborder contains only two different families, the solitary (or Colloidea monozoa) and the associated  (or Colloidea polyzoa). Both families are very nearly allied, and differ only in one single essential character: the solitary life of the former, the social union of the latter. It seems to be merely in consequence of this difference that the cleavage of the nucleus commonly takes place very late in the former, very early in the latter.

Therefore the full-grown Thalassicollida (till immediately before propagation) commonly exhibit one single nucleus in the centre of the capsule, whilst in the Collozoida the capsule is distended by numerous small nuclei. In these latter the centre of the capsule usually contains one large oil-globule, whilst in the former oil-globules are either wanting, or scattered in large numbers in the endoplasm, or disposed in one layer on the inside of the capsule membrane.

In the solitary Thalassicollida each capsule is enclosed in its own peculiar spherical calymma, whilst in the associated Collozoida all capsules of the colony are united in one common, very voluminous calymma.

Thalassicollida, Haeckel, 1862, Monogr. d. Radiol., p. 246. Thalassicollida, Haeckel, 1881, Prodromus, p. 469.

Definition.— solitaria.

The family Thalassicollida comprises all solitary without a skeleton. The oldest and best known form of this family is the genus Thalassicolla, as restricted by