Invertebrates do not have bony skeletons like vertebrates, however, many have an inner or exterior skeleton of some kind which is made from numerous supplies. Some invertebrates keep their physique form by a troublesome, versatile cuticle and excessive inner strain.
Among lesser phyla of invertebrates are the Hemichordata, or acorn worms, and the Chaetognatha, or arrow worms. Other phyla embody Acoelomorpha, Brachiopoda, Bryozoa, Entoprocta, Phoronida, and Xenoturbellida. The annelids comprise the segmented worms, with about 15,000 fashionable species, together with the properly-recognized earthworms and leeches. They are present in most wet environments, and include many terrestrial, freshwater, and especially marine species (such as the polychaetes), in addition to some which are parasitic or mutualistic.
The external layers of aquatic invertebrates are typically skinny and are permeable to water, permitting the change of gas, though some have specialised respiratory structures on their physique surface. Aquatic invertebrates feed by ingesting … Read MoreRead More »