Environmental biology is one of the critical Biology Topics that involves understanding how humans impact the environment and how to address environmental issues.
Introduction to Porifera
Porifera (Gr., porus-pore; ferre – to bear; ‘organisms with holes’)
1. Sessile (stalk-less), sedentary (attached to the substratum), and marine except for one group that lives in freshwater. These are non-motile animals attached to some solid support. They are commonly called sponges.
2. Simplest multicellular, diploblastic animals.
3. The body design involves minimal differentiation and division into tissues. The cells are loosely held together in a gelatinous matrix, mesoglea, or mesohyl, and do not form tissues.
4. Asymmetrical or radially symmetrical. Sponges may be vase-like, rounded, sac-like, or branched.
Differences between Bilateral Symmetry and Radial Symmetry of Animals
| Bilateral Symmetry | Radial Symmetry |
| 1. Limbs and organs are paired. | 1. Limbs and organs occur all around the central axis. |
| 2. Cephalization is present. | 2. Cephalization is absent. |
| 3. The animal’s body can be divided into two equal halves (i.e., mirror images) by one plane called the mid-sagittal plane. | 3. The animal’s body can be divided into two equal halves by any vertical plane passing through the central axis. |
| 4. Examples: Earthworms, fish, frogs, human beings, etc. | 4. Examples: Hydra, starfish, etc. |
5. Body is perforated with numerous pores. The Ostia opens into a canal system having canals and chambers lined with collared flagellated cells or choanocytes. It also has one or more large-sized outlets for water, called oscula (singular osculum). The canal system is a filter-feeding system of sponges. It helps in circulating water throughout the body to bring food, oxygen, and water.
6. Mouth, digestive cavity, and anus absent.
7. Skeleton is made up of minute calcareous or silicious spicules or spongin fibre (collagen) or both.
8. Reproduction is both asexual by budding and gemmules and sexual through fertilization. Possess the great power of regeneration.
Phylum Porifera is divided into the following three classes:
Class 1. Calcarea – Marine
Examples: Leucosolenia, Sycon (Scypha)
Class 2. Hexactinellida – Marine
Examples: Ewplectella (Venus’s flower basket), Hyalonema (rope sponge).
Class 3. Demospongiae – Mostly marine and a few freshwater solitary or colonial forms.
Examples: Cliona (boring sponge), Spongilla, and Euspongia (bath sponge).
