By Diana R. Kershaw (auth.)
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Additional resources for Animal Diversity
The larger macronucleus is kidney-shaped with the smaller micronucleus lodged in a depression at one side. The macronucleus controls the ordinary activities of metabolism and growth while the micronucleus is concerned with reproduction. Thus if the micronucleus is removed the animal can survive, but not reproduce. Locomotion The animal moves through the water by beating its cilia, and may also use them to glide over a solid substrate. In slow motion a cilium may be seen to stiffen and bend over rapidly to lie nearly parallel with the body surface.
Each individual is vase-shaped with a central cavity (the paragaster or spongocoele) and a large opening (the osculum) at the opposite end of the attachment (stolon). The sponge wall consists of three layers: an outer pinacoderm; a supporting gelatinous protein matrix, the mesohyal; and an inner feeding layer of choanocytes, which are closely similar to the choanoflagellates. The pinacocytes are highly contractile, and thus the animal is able to increase or decrease in size. Further contract ability is provided in some species by the collenocytes, which have long cytoplasmic strands extending across the spaces through The protozoan ciliates demonstrate the greatest complexity that has evolved in a unicellular organism.
Since sponges have no obvious protective devices and no ability to escape they might be regarded as particularly vulnerable to predators. In fact they are avoided by most potential predators such as fish, since their sharp spicules will penetrate any soft tissue exposed to them and they apparently taste and smell unpleasant. Indeed many small animals, for example crabs, take advantage of this to protect themselves. For example, some small marine sponges are always found in association with hermit crabs.