Invertebrate Zoology: Study Guide - T. A. Dauda 2014

Multicellular Animals
Annelids
Class Polychaeta, or Polychaetes

Polychaete Annelids comprise about 5,300 species (Fig. 29). They inhabit predominantly aquatic environments. Most of them are marine benthic animals that crawl along the bottom or burrow into silt and sand, mainly in the coastal zone. Some polychaetes lead a sessile lifestyle, attaching themselves to rocks, mollusk shells, etc. At the same time, many of them hide in tubes formed from hardened organic or calcareous Skin secretions. A number of species swim freely in the plankton. A small number of freshwater representatives, found in Lake Lake Baikal, and parasitic members of the Class are also known.

Fig. 29 Species of polychaete annelids:

A — lugworm (Arenicola); B — clam worm (Nereis); C — Phyllodoce; D — Amphitrite; E — Sabellaria; F — Serpula; G — sea mouse (Aphrodite); 1 — tentacles; 2 — gills; 3 — parapodia.

Fig. 30 External Structure of Nereis:

A — HEAD region; B — trunk region; C — anal region: 1 — tentacles; 2 — palps; 3 — prostomium; 4 — peristomium.

The length of polychaetes ranges from a few millimeters to 3 m. Most of them have an elongated, cylindrical, or slightly dorsoventrally flattened worm-like shape (Fig. 30). The body of these worms is divided into head, trunk, and anal regions.

The head region includes 2 segments. The first of these is the head lobe (prostomium), flattened dorsoventrally and pyramidal in shape. Its narrow anterior end bears 2 small appendages — tentacles, which serve as Organs of Touch. On the sides of the head lobe, from its base, extend a pair of more massive tactile appendages — palps. On the DORSAL SIDE OF the head lobe are the eyes and olfactory pits. The second segment, the oral segment (peristomium), is wider and longer than the head lobe. The Mouth is located on its lower side, and several pairs of cirri are on the sides.

The trunk region consists of segments, or rings. In most species, the sides of each trunk segment bear mobile body-wall outgrowths set with setae. These are the organs of locomotion — parapodia (Fig. 31); with their help, polychaete annelids crawl and swim.

Fig. 31 Parapodia of Nereis:

1 — dorsal cirrus; 2 dorsal lobe; 3 ventral lobe; 4 ventral cirrus.

The last, anal region is represented by a single segment. The anus opens on it. Limbs are absent. Thus, in polychaetes, for the first time among invertebrates, The formation of a distinct head region (cephalization) and The Development of various Sense Organs on it are observed. Furthermore, parapodia, or primitive limbs, appear in them for the first time. They are regarded as the precursor of more advanced limbs in higher invertebrates. Apparently, jointed arthropod limbs evolved from these primitive appendages through complexification in the course of evolution.

The body of polychaete annelids is covered by a skin-muscular sac consisting of a single-layered epithelium with a thin cuticle On the surface and 2 Muscle layers: outer circular and inner longitudinal, which provide body contraction and bending.

The Digestive System begins at the anterior end of the body with the mouth and ends with the anus at the posterior end. The intestine consists of 3 sections: foregut, midgut, and hindgut.

The Nervous system is represented by a circumesophageal nerve ring consisting of supraesophageal and subesophageal ganglia connected by nerve cords. Two nerve trunks extend from the subesophageal ganglion, bearing thickenings — ganglia — in each segment, which are interconnected by commissures. Consequently, the entire nervous system resembles a ladder. Of the sense organs, polychaetes have well-developed eyes, tentacles, cirri, and statocysts.

The Circulatory system is closed. It consists of dorsal and ventral vessels that merge into each other at the anterior and posterior ends of the body. Blood movement is driven by rhythmic contractions of the dorsal vessel and a series of circular vessels in the anterior part of the body.

The respiratory organs of many marine annelids are gills, which are cutaneous outgrowths of various shapes. In others, Respiration occurs through the skin, which is rich in Blood Vessels.

The excretory system is represented by metanephridia. These are paired, convoluted tubules that begin with a ciliated funnel on the walls of the body cavity and end with an excretory pore opening to the outside.

Most polychaetes are dioecious, with very few being hermaphrodites. Fertilization is usually external. Development typically proceeds with several larval stages.

Many polychaete annelids serve as a primary food source for fish and therefore play a major role in the biogeochemical cycling of matter in the biosphere. The distribution and Abundance of polychaetes are taken into account when breeding commercial fish species. For instance, attempts to introduce valuable Black Sea fish species into the Caspian Sea failed due to the Caspian's poverty in polychaetes. In the 1930s, under the leadership of L. A. Zenkevich, about 65,000 clam worms were introduced into this enclosed sea. Within a few years, these worms multiplied in large numbers and became a favorite food of sturgeons. The palolo worm is consumed as food by the inhabitants of Pacific islands.