Porifera
Porifera also known as a sponge is one of the most primitive multicellular aquatic animals. It lacks a respiratory organ and oxygen is supplied by a direct exchange between the tissues and surrounding water. It also has absence of a nervous system because it has lack of coordination in the oscula, pores, and choanocytes so the slow reaction of sponges to stimuli shows no nervous system. Sponges are also filter feeders; water is drawn into the spores tiny hair like flagella, passes through small pores and into the central cavities. Some Sponges start sexual reproduction when only a few weeks old while other wait till several years old. They reproduce sexually and asexually meaning only one gender at a time. They have no permanent gonad, instead number of areas of the sponge will during the reproduction period become changed to produce either sperm or egg.
Nematode
Nematodes (also known as roundworms) exist as parasites or as free living organisms and play a role as decomposers that break down organic materials to be utilized by bacteria. They have a worm structure with a lack of features such as cilia or a well defined head. They have an internal body cavity called pseudocoelom which looks like a tube that runs the whole length of their bodies, which also contains the intestines and reproductive organs of the roundworms. Nematodes have a nervous system contains nerves extending to the mouth and the length of the roundworms. There are two nerve cords to activate the muscles relalying sensory information. Nematodes contains a few small organs and a pharynx where food is pulled in and crushed and then moves to the gut cavity, where nutrients and wastes are spread throughout the body cavity by diffusion and taken out by the excretory canal tube. These roundworms have separate sexes when males use a special spine to inject sperm into the females gonophore, they lap up to 27 million eggs at a time.
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Mollusks
Mollusks are soft bodied invertebrates including snails, slugs, octopus, squid, clams, and mussels. Most of them have shells to protect them. Most mollusks have a muscular foot for creeping for burrowing. Some also have a head with sense organs. The soft body has lungs or gills for breathing and digestive and reproductive plants. Most mollusks have a rasping tongue call radula with tiny teeth which help scrapes its food into tiny chunks. Mollusks reproduce sexually. Slugs and snails possesses both female and male organs but still needs to mate for fertilization. Most lay eggs that hatch into small free swimming larvae. Mollusks have shells in many shapes and sizes to provide somewhere to hide and to prevent the moist soft body from drying out.
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