The large central opening in the parazoan body is called the:

a. gemmule.

b. spicule.

c. Ostia.

d. osculum.

The simplest invertebrate classification, known as parazoans, only includes the phylum Porifera, or sponges. Gemmules are strong outer layers that enclose clusters of cells. These features, which are not the central aperture found in the bodies of Porifera, aid in asexual reproduction in freshwater sponges (Porifera).

The simplest sponges have a morphology that resembles an uneven cylinder with a big central cavity inside, called a spongocoel. Numerous ostia, or pores, which form openings in the body wall, allow water to enter the spongocoel. Water that enters the spongocoel is discharged through a sizable communal orifice known as the osculum.

Protein fibres called spicules give sponges (Porifera) their tensile strength. The mesohyl secretes these fibres, which resemble small needle-like structures. Sponge (Porifera) have canals called ostia that enter their interior through the mesohyl.