Phylactolaemata sentence example
- The Phylactolaemata are a small group confined to fresh water, and possess clear indications of adaptation to that habitat.
- In the Phylactolaemata, however, a new colony may originate not only from a larva, but also from a peculiar form of bud known as a statoblast, or by the fission of a fully-developed colony.
- The form of the colony may thus be a good generic character, or, on the contrary, a single genus or even species may assume a variety of different forms. While nearly all Polyzoa are permanently fixed to one spot, the colonies of Cristatella and Lophopus among the Phylactolaemata can crawl slowly from place to place.
- The lophophore is a simple circle in all Polyzoa except in the Phylactolaemata, where it typically has the form of a horse shoe outlined by the bases of the tentacles.
- This is continuous with the general body-cavity in the Phylactolaemata, while in the Gymnolaemata it develops in the bud as a part of the body-cavity, from which it becomes completely separated.Advertisement
- In the Phylactolaemata the outermost layer of the bodywall is a flexible, uncalcified cuticle or "ectocyst," beneath which follow in succession the ectoderm, the muscular layers and the coelomic epithelium.
- Thus in the Phylactolaemata the contraction of the muscular body-wall exerts a pressure on the fluid of the body-cavity and is the cause of the protrusion of the polypide.
- Finally, in the Phylactolaemata, the larva becomes an ancestrula before it is hatched, and one or several polypides may be present when fixation is effected.
- On the view that the Phylactolaemata are nearly related to Phoronis (see Phoronidea), it is extremely difficult to draw any conclusions with regard to the significance of the facts of development.
- If the Phylactolaemata were evolved from the type of structure represented by Phoronis or the Pterobranchia, the Gymnolaen ata should be a further modification of this type, and the comparative study of the embryology of the.two orders would appear to be meaningless.Advertisement
- It seems more natural to draw the conclusion that the resemblances of the Phylactolaemata to Phoronis are devoid of phylogenetic significance.
- The supposed occurrence of a pair of nephridia in certain Phylactolaemata, in a position corresponding with that of the nephridia of Phoronis, must also be mentioned, although it has been maintained that the "nephridia " of Phylactolaemata are merely ciliated portions of the body-cavity and not indeed nephridia at all.
- But a serious objection to the comparison is that the development of Phylactolaemata can be explained by supposing it to be a modification of what occurs in other Polyzoa, while it appears to have no relation whatever to that of Phoronis.