Level Of Organization
Sponges are multicellular, but the cellular level of organization means that the cells do not form organized tissues
They have Three germ layers (endoderm, mesoderm and ectoderm) - they are all triploblasts (like us).
The head of a nematode is relatively distinct. Whereas the rest of the body is bilaterally symmetrical, the head is radially symmetrical.
They donot contain cephalization.
Rather, nutrients and waste are distributed in the body cavity, whose contents are regulated by an excretory canal along each side of the body.
They are very abundant and occur almost everywhere. They are not segmented. Do not confuse them with earthworms.
The nematode digestive system is generally divided into three parts, the stomodeum, intestine, and proctodeum.
Donot contain a circulatory system.
Does not contain a rspiratory system.
Nematodes have a unique excretory system consisting, in simpler species, of one or two one-celled glands called renette cells.
Four peripheral nerves run the length of the body on the dorsal, ventral, and lateral surfaces. Each nerve lies within a cord of connective tissue lying beneath the cuticle and between the muscle cells. The ventral nerve is the largest, and has a double structure forward of the excretory pore. The dorsal nerve is responsible for motor control, while the lateral nerves are sensory, and the ventral combines both functions.
Most nematode species are dioecious, with separate male and female individuals. Both sexes possess one or two tubular gonads. In males, the sperm are produced at the end of the gonad, and migrate along its length as they mature. The testes each open into a relatively wide sperm duct and then into a glandular and muscular ejaculatory duct associated with the cloaca. In females, the ovaries each open into an oviduct and then a glandular uterus. The uteri both open into a common vagina, usually located in the middle of the ventral surface.