Polychaeta
Morphology

The body plans of polychaete worms often reflect their habits and habitats. This is shown in the great diversity of structural forms that deviate from the basic metameric body. The basic polychaete body plan includes the following: (1) a head consisting of a prostomium and a peristomium; (2) a segmented body or metastomium; and (3) the anal segment or pygidium. Variations of the morphological characteristcs of these parts of the body are important for the identification of polychaetes. The holoplanktonic species are highly specialized for pelagic life, and the main morphological characters used for their identification will be described briefly.

The basic body plan of a pelagic polychaete is illustrated in the following figures:

body plan pelagic polychaete
anterior region ventral view
type of parapodial gland
type of parapodial gland 2
type of parapodial gland 3
types of setae
type of setae 2

The head presents a prostomium with a variable number of antennae (1, 2, 4, 5, 6), eyes with different degrees of development, and nuchal organs which appear as ciliated pits, grooves or lobes with different forms in typhloscolecid polychaetes.

Travisiopsis dubia
Travisiopsis lanceolata
Travisiopsis levinseni
Travisiopsis lobifera
Travisiopsis lumbricoides
Typhloscolex mulleri

The peristomium of the head is usually integrated by fusion of some segments with well- developed food-gathering appendages referred to as tentacular cirri (anterior region ventral view). There is often an eversible buccal cavity or proboscis that may have chitinous jaws (Phalacrophorus uniformis 2).

Proboscides are variable in size and in the distribution of their papillae:

Rhynchonerella gracilis
Vanadis crystallina
Vanadis minuta
Vanadis tagensis
Vanadis studeri

In the metastomium each segment of the body bears a pair of lateral parapodia. These structures are typically biramous with the parapodial trunk dividing into a dorsal ramus, the notopodium, and a ventral ramus, the neuropodium. This biramous condition is present in tomopterid polychaetes, but in the other holoplanktonic families the parapodia are uniramous (type of parapodial gland). Each ramus consists of a setigerous lobe supported by a stout internal chitinous rod or aciculum, and a bundle of chitinous setae which show an infinite variety of forms.

Naiades cantraini 2
Watelio gravieri 2
Vanadis antarctica 4
Maupasia coeca 2
Lopadorhynchus henseni 3

Setae are also referred to as chaetae by some zoologists. Holoplanktonic families present two main kinds of setae: simple and compound. Simple setae are capillary setae (slender hair-like structures, and simple acicular setae (stout, similar to the aciculum. Compound setae consist of a basal and distal parts. The distal part may be flattened to form a blade, and may have serrations. Compound setae may also be acicular, as in some species of Rhynchonerella. Fauchald (1977) has remarked on the importance of accurate examination of these structures through precise microscopic observations (types of setae).

Each parapodium may bear dorsal and ventral structures named parapodial cirri that are variable in form and size. In addition, some glands are associated with the parapodia, and they have taxonomic importance in the holoplanktonic families Alciopidae and Tomopteridae. In the Alciopidae, the segmental glands or organs are located laterally, dorsally or ventrally between the parapodial lobes on most body segments.

Alciopa reynaudi 2
Rhynchonerella angelini 2
Plotohelmis capitata 2
Vanadis crystallina 2
Vanadis formosa 2
Vanadis studeri 2 , male
Vanadis studeri , female

Usually they are pigmented and their form and pigmentation pattern are used in the identification of some species (Vanadis longissima). In the Tomopteridae, there are various types of parapodial glands located in the notopodial and neuropodial pinnules which are membranous structures adapted for swimming. Chromophil glands are always present in all species, and occur only in the ventral or distal regions of neuropodial pinnules, from the third parapodia on toward posterior. They are often swollen and can be stained by haematoxylin. Spur glands are present in some species. They are alone in the first two or three parapodia, but associated with chromophil glands in the subsequent parapodia. In some species, rossete glands are present in the trunks of the first two parapodia and in the notopodial and neuropodial pinnules of later parapodia. Hyaline glands occur near the apex of the pinnule as clear areas around a yellowish spot. These structures are very difficult to see because the spot is often missing. Day (1967) recommends a brief staining with haematoxylin to make the clear area readily visible. This basic plan of the parapodia can be modified whereby any part of the parapodium may be suppressed. All the setae may be lacking (achaetous) as in tomopterid polychaetes (type of parapodial gland ,type of parapodial gland 2 ,type of parapodial gland 3).

The setigerous lobes may not be present, leaving only dorsal and ventral cirri (apodous), as the first parapodia in the genus Vanadis (Vanadis crystallina, Vanadis studeri). The cirri may be lost, such as the ventral cirrus in the first parapodia in some species of the genus Lopadorhynchus (Lopadorhynchus henseni 2 , Lopadorhynchus appendiculatus 2), or the cirri may be well developed, as in all species of the family Typhloscolecidae (Travisiopsis coniceps , Travisiopsis lanceolata).

Finally, the pygidium which is the last segment of the body that bears the anus, only conserves a pair of cirri named anal or pygidial. They are taxonomically important only in the family Typhloscolecidae (Travisiopsis dubia 2 , Travisiopsis lanceolata 2 , Travisiopsis levinseni 2 , Typhloscolex mulleri 2). In the Tomopteridae, the pygidial region may be elongated bearing rudimentary parapodia and receives the name of tail (Tomopteris nationalis).

Several sources of polychaete terminology were used for definitions, including Fauchald (1977), Pleijel and Dales (1991), and Glasby (1996).