Sponges = simplest multicellular animals
No gills, lungs, or trachea — just water flowing through canal system
All cells close to water → gas exchange by simple diffusion
O2 diffuses IN from water; CO2 diffuses OUT into water
Answer: Simple diffusion
Phylum Porifera: pore-bearing, sessile (fixed), aquatic (mostly marine), asymmetrical or radially symmetrical, diploblastic (2 cell layers: pinacoderm outside, choanoderm inside). No true tissues, organs, or nervous system. Reproduction: sexual (larvae swim, settle, become adults) + asexual (budding, fragmentation, gemmules).
Water pathway: Ostia (pores) → Incurrent canals → Flagellated chambers (choanocytes) → Excurrent canals → Spongocoel → Osculum. Choanocytes beat flagella creating current. Functions: gas exchange, nutrient uptake, waste removal, reproduction (sperm and egg release).
Skeleton of sponges: Calcareous sponges: calcium carbonate spicules. Siliceous sponges (glass sponges): silica spicules. Horny sponges: spongin fibres (protein). Bath sponges (Euspongia, Hippospongia): only spongin, soft. Spicules provide structural support.
Sponges are filter feeders — they pump enormous volumes of water, filtering bacteria and small particles. Ecological role in marine nutrient cycling. Bath sponges (dried spongin skeleton) used historically. Sponges produce bioactive compounds (anti-cancer, anti-bacterial) of pharmaceutical interest.
Water enters through minute pores called ostia, passes through a system of canals into the central spongocoel, and leaves by a single large opening, the osculum. This one-way current brings in food and oxygen and carries out waste and gametes. Choanocytes, or collar cells, line the canals and their flagella drive the current while their collars trap food particles. Digestion is entirely intracellular — sponges have no digestive cavity.
Poriferans are multicellular but have no true tissues, so they are described as being at the cellular level of organisation. They have no nervous system, no muscles, no organs and no body symmetry in most species. The body is supported by spicules of calcium carbonate or silica, or by spongin fibres. This absence of tissues is the single feature that separates Porifera from every other animal phylum.
Sponges reproduce asexually by fragmentation and by forming gemmules, internal buds that survive unfavourable conditions. Sexually they are mostly hermaphrodite, producing both eggs and sperm, but fertilisation is internal and cross-fertilisation is the norm because eggs and sperm mature at different times. Development is indirect, with a free-swimming larval stage that is morphologically distinct from the adult.
Calling the spongocoel a digestive cavity. It is a water canal. Digestion happens inside individual cells, not in the cavity.
Saying sponges have tissues. They are multicellular but at the cellular level of organisation — cells are not organised into true tissues.