Invertebrate Zoology: Study Guide - T. A. Dauda 2014

Metazoa
Sponges
Class Demospongiae

Most modern sponges belong to this Class. Their Skeleton is siliceous, spongin-based, or a combination of both.

Representative species include large globular geodias (Geodia), brightly colored orange-red sea oranges (Tethya), boring sponges, bath sponges, as well as a few representatives of freshwater sponges, such as bodyas (badya).

Phylogeny of the Phylum Porifera

The Organization of sponges exhibits many signs of deep primitivity: the absence of true differentiated Tissues and Organs, the extreme plasticity of cellular elements, and a lack of sharply defined individuality within colonies. All these indicate that sponges are the simplest structured Multicellular Organisms.

Recalling Mechnikov's theory on THE ORIGIN OF Multicellular animals, it is easy to see that the larva typical of most sponges—the parenchymula—almost perfectly matches the hypothetical Mechnikov phagocytella. It features a superficial ectodermal layer of flagellated Cells and a loose internal layer of cells, the endoderm.

It can be hypothesized that the phagocytella transitioned to a sessile lifestyle, thereby giving rise to the phylum Porifera. In doing so, The Fate of the phagocytella's Cell layers in sponges turned out differently than in all other multicellular animals. The outer layer of flagellated cells in sponges gave rise to the digestive layer of choanocytes (the endoderm, rather than the ectoderm as in other multicellular organisms), whereas the inner layer of cells, which usually forms the endoderm, transformed into the body surface cells—the ectoderm and the cellular elements of the mesohyl.

Some zoologists believe that sponges evolved from colonial choanoflagellates independently of other multicellular animals. Other researchers suggest that multicellular organisms share a common trunk from which sponges separated very early. The second view appears more grounded because the sponge larva—the parenchymula—is similar to the planula of Cnidarians, pointing to a common origin.

Sponges are very ancient organisms. Researchers find their numerous fossil remains in Cambrian marine deposits, and they are also encountered in Proterozoic rocks.

QUESTIONS FOR SELF-assessment and review:

1. How do multicellular animals differ from unicellular ones?

2. Where do sponges inhabit?

3. What types of sponge body Structure exist?

4. How do sponges reproduce?