External Morphology

The body of a polychaete consists of three basic regions; the presegmental region (comprised of the prostomium and peristomium), the segmented body (including parapodia, chaetae and segmentally arranged internal organs) and the post-segmental pygidium (consisting of the mesodermal growth zones and the pygidial cirri). The body of Perenereis spp. is covered by a thin, flexible, external cuticle which is used for body support as well as for defence.
                               
                                

diagram sourced from Ruppert et al., 2004, pp. 424

	The head region of Perenereis spp. is very distinct. The head consists of two antennae, two palps, four ocelli (simple eyes) and four pairs of tentacular cirri. Although the palps and antennae may look similar in some species, the palps usually extend further forward, are located ventrally and have a more robust structure than antennae (Beesley et al., 2000). It is likely that the ocelli of this organism are capable of detecting light and it's direction but are unable to form definitive images. The tentacular cirri and tentacular palps have migrated from the prostomium, elongated and have assumed a feeding function. this progression can be observed during the process of anterior regeneration whereby anteriorly located parapodia migrate to the most anterior segment of the body and become specialised (for more information see ‘Cell Biology’). most of the appendages described above for the errant Perenereis are well-developed for a mobile existence and are not present or are considerably modified in tube-dwelling and entirely sedentary species.
	The pharynx of this species is tubular, muscular and eversible and contains a pair of stout jaws.
	The trunk of Perenereis can be extremely long and elongate. This species may reach up to 20 cm in intertidal environments (Beesley et al., 2000). Gills are not present in this species.
	Each segment of a polychate worm has a single pair of parapodia. Each parapodia consists of an upper notopodium and a lower or ventral neuropodium. Both of these are supported by an internal chitinous rod – the aciculum. Muscles attach to these skeletal rods to attain movement. Two sensory cirri project from both the dorsal and ventral surface of the notopodium and neuropodium respectively.
	One pair of pygidial cirri located at the furthermost posterior region of the body on the pygidium.

Parapodia may be apparent in various forms in different species based on their environmental ecology and the structure of individual parapodia changes from the anterior to posterior regions of the organism. 

Anterior parapodia
Mid-section parapodia
Posterior parapodia

Nereis spp. has three fleshy divisions of the parapodium. The aciculi are visible under a high-powered dissecting microscope. Bundles of chaetae protrude from the parapodia to improve the polychate's grip on the substratum. The tips of the chaetae have been modified by individuals for various functions. Shapes include hooks, needles and serrated ends for gripping the substrate or paddle-like for digging and swimming. The chaetae shape of the Perenereis spp. were not visible under the dissecting microscope. The parapodia and mouthparts are commonly used to identify polychaete species.

Segments form sequentially during development and can be added throughout the life of the polychaete although some have a maximum number of segments which can be acquired. The growth zones are located in the post-segmental pygidium. New segments are produced on the anterior region of the pygidium and thus the youngest part of the body is the most posterior segment (Schroeder & Hermans, 1975).