Porifera

From Palaeos

Choanomonada
`--Metazoa
   `==Porifera (paraphyletic?)
      `==Radiata (paraphyletic?)
         `--Bilateria
            `==Acoelomorpha (paraphyletic?)
               |--Protostomia
               |  |--Ecdysozoa
               |  `--Lophotrochozoa
               `--Deuterostomia

An orange Elephant Ear sponge, Agelas clathrodes (Demospongiae, Ceractinomorpha, Agelasida, Agelasidae) at the Florida Keys National Maritime Sanctuary.

The most primitive form of multicellular animal life, it is thought that sponges evolved independently of metazoan animal life, or else of higher metazoa. A recent alternative theory, suggested through molecular phylogeny is that they represent a paraphyletic grade of basal metazoa. Regardless of their actual phylogenetic relationship, the Porifera in many respects they are still little more than colonial choanoflagellate protozoa. Over 5000 recent species are known, and many more fossil species. All are benthic, sessile, suspension-feeders which inhabited a wide variety of exclusively marine environments, from the early Cambrian to the present day.

Sponges make their skeleton out of both organic fibers (primarily a material called spongin (the stuff of natural bath sponges), secreted by spongocyte cells, and inorganic spicules (calcium carbonate or opalline silica needle-like structures), secreted by sclerocyte cells.

The major fossil record of these animals consists of the resistant spicules. After death, spicules are scattered across the sea floor and may be found as disarticulated microfossils.

o Metazoa 
`--o PORIFERA
   |--Hexactinellida 
   `--+--+--Demospongiae 
      |  `?-Archaeocyatha
      |==Sclerospongiae (polyphyletic)?
      |==Stromatoporoidea (polyphyletic)?
      `--+--Calcarea 
         `--Radiata / Eumetazoa

Classification

There are four main classes:

Class Calcarea

Class Demospongiae

Class Sclerospongiae

Class Hexactinellida

Two fossil groups are often included - the Stromatoporoidea and the Archaeocyatha. The former are most likely Porifera (possibly Sclerospongiae or Demospongiae), while the precise relationships of the latter remain controversial.

Organization of the Poriferan Body Plan

The basic body form of sponges consists of numerous small incurrent canals called ostia (sing. ostium) and one or more large excurrent opening called oscula (sing. osculum). Organization of chambers and channels varies however from simple to quite complex.

Credits

MAK030430