Question

In the most simple type of canal system of Porifera, which of the following ways exhibit water flow?
a. Ostia $\rightarrow$ Spongocoel $\rightarrow$ Osculum $\rightarrow$ Exterior
b. Spongocoel $\rightarrow$ Ostia $\rightarrow$ Osculum $\rightarrow$ Exterior
c. Osculum $\rightarrow$ Spongocoel $\rightarrow$ Ostia $\rightarrow$ Exterior
d. Osculum $\rightarrow$ Ostia $\rightarrow$ Spongocoel $\rightarrow$ Exterior

Answer 1

Porifera are pore bearing animals and commonly called sponges. They have cellular organization, cell aggregate plan and are mostly asymmetrical. They are mostly marine but about 150 sponge species inhabit fresh water, e.g. Spongilla.

Complete answer:
Porifera have an endoskeleton of either calcareous spicules or siliceous spicules or of sponging fibres.

They are divided into three classes based on the endoskeletal elements:

Class Calcispongiae or Calcarea – They have an endoskeleton formed of calcareous spicules, e.g. Scypha. They have a simple canal system and they do not occur at depths of sea.

Class Hexactinellida or Hyalospongiae- Their endoskeleton is formed of six-rayed siliceous spicules, e.g. Hyalonema (Glass rope sponge), Euplectella (Venus flower basket).

Class Desmospongiae- Their endoskeleton is formed of either sponging fibres or other siliceous spicules or both, e.g. Euspongia (common bath sponge), Cliona (Boring sponge).
They have basically two types of pores- numerous and smaller ostia and one or few larger oscula which act as inlets and outlets of water currents.

They have a peculiar canal system whose central cavity is called paragastric cavity or spongocoel. The simplest type of canal system in Porifera exhibits this type of water flow: Ostia $\rightarrow$ Spongocoel $\rightarrow$ Osculum $\rightarrow$ Exterior. The water current flowing through the canal system helps in nutrition, respiration, excretion and reproduction.

Hence, the correct answer is option (A).

Note: Porifera are diploblastic animals and their body wall is formed of two layers- outer dermal layer or pinacoderm of flat pinacocytes and inner gastral layer or choanoderm of flagellated collar cells or choanocytes whose flagella beat and maintain a water current.