Invertebrate Excretory and Digestive Systems
Excretory Organs in Invertebrates
1. Protonephridia
Characteristics of flatworms. They are highly branched tubes whose inner end terminates in a cell with flagella, called a flame cell, and the outer end terminates in an excretory pore. The flame cells are among the intercellular fluids.
2. Metanephridia
Typical of annelids and mollusks. The metanephridia are long tubes with an inner end shaped like a funnel (nephrostome) having cilia. The coelomic cavity communicates with the anterior segment. The movement of the cilia drives the liquid into the funnel. Along the tube, needed substances are reabsorbed, leaving only a watery fluid with waste being disposed to the outside through an excretory pore.
3. Antennal and Maxillary Glands
These are the excretory organs of crustaceans. The glands are paired and are surrounded by blood. They consist of a glandular chamber, which includes filtering, an excretory tube, where reabsorption of certain substances occurs, a muscular bladder, and an excretory pore that connects to the outside.
4. Malpighian Tubules
These are the excretory organs of insects, millipedes, and some arachnids. They are thin tubes that are distributed throughout the body. They have a blind end and another open to the intestine, where they dump the waste products.
Different Digestive Systems of Invertebrates
1. Porifera
They have an epidermis with inhalant pores through which water and food particles enter into an atrium, where the particles are taken up by cells via phagocytosis. The water comes out of the osculum. Digestion is intracellular.
2. Cnidaria
There is a gastrovascular cavity with a single opening, which acts as both mouth and anus. Around the opening are tentacles with stinging cells that help capture prey. All polyps are carnivorous and their digestion is mixed.
3. Platyhelminthes (Flatworms)
They have only one central and ventral mouth, muscular pharynx, and a highly branched, blind gut. Undigested wastes are expelled through the mouth. Parasitic flatworms have no digestive tract and absorb nutrients directly through the skin.
4. Annelids
They have a regionalized digestive tract. Some annelids have a pharynx with a suction function. In the earthworm, its parts are: mouth, pharynx, esophagus, crop, muscular gizzard, intestine, and anus.
5. Mollusks
They have a gut with attached glands. Some groups have a rasping organ called a radula. Salivary glands empty into the pharynx, the esophagus has cilia, the stomach is large and is connected to the hepatopancreas (which pours in digestive enzymes), and the intestine ends in the anus.
6. Arthropod – Crustaceans
It has a mouth, a short esophagus, and a stomach with two parts: one for chewing (the cardiac sac) and another for digestive functions (the pyloric sac). The intestine is long and ends at the anus. It has a large hepatopancreas.
7. Arthropod – Insects
They have a mouth, pharynx, crop, gizzard, stomach with gastric caeca, intestine, and anus. Their mouthparts are highly modified for their food, adapted to the foods they consume.
8. Echinoderms
The mouth is ventral with a short esophagus and stomach. The intestine ends at the anus.