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Arenicolidae: Arenicola bombayensis

The prostomium is small and rounded or conical. The peristomium is typically reduced to lips around the mouth, but some genera such as Branchiomaldane appear to have a complete peristomial ring. Antennae and palps are absent. Nuchal organs are paired dorso-lateral pits on the prostomium. Longitudinal muscles are in four distinct bands and the body segmentation is distinct. The first segment is similar to the other segments and has no parapodial structures in adults; tentacular cirri are absent. Notopodia are short, distally truncate cylinders or cones, and the neuropodia are present as tori. Dorsal and ventral cirri are absent. Branchiae are present as branching tufts associated with notopodia in the mid-section of the body in most taxa and as simple, tapering structures in Branchiomaldane. Epidermal papillae are absent. Pygidial papillae are absent. Lateral organs are absent; dorsal cirrus organs have not been observed and are presumably absent. The simple axial pharynx (= proboscis) is sac-like and covered externally with large papillae. A gular membrane is present, and the gut is a straight tube. The segmental organs are mixonephridia; only a few pairs are present anteriorly, the first pair is purely excretory. The circulatory system is closed, and a heart body is absent. Aciculae are absent. Chaetae are present as variously ornamented notopodial capillaries and as distally dentate, unprotected neuropodial hooks.

The above description is based on Hutchings (2000), which in turn is based on Fauchald & Rouse (1997).

Identification tips

Recognising the family
Many representatives of the Arenicolidae are very characteristic with robust bodies divided into 3 sections, a thorax with an eversible proboscis, followed by a middle section with large branched tufted branchiae from chaetiger 7 and a posterior trunk with no chaetae. The epidermis is thick and rugosecrenulate. Species belonging to Branchiomaldane superficially resemble capitellids, with an anterior region of abranchiate segments and a posterior region with simple branchial filaments, lacking an achaetous tail region, with long shafted dentate hooks which lack hoods. (The capitellids which superficially resemble Branchiomaldane are typically smaller with smooth epidermis and with body divided into only 2 sections and branchiae if present are much smaller. Also the structure of the hooded hooks differs completely, arenicolids having simple dentate hooks whereas capitellids have hooded hooks.)

Scalibregmatidae, like Arenicolidae, have thick rugose crenulated epidermis but scalibregmatids have a T shaped prostomium which is absent in arenicolids.

Distinguishing species
Identification to species may require dissection of the body wall to reveal the distribution of the nephridia and the segment on which they open. Species of Branchiomaldane have simple tapering branchiae which begin from chaetiger 18; members of this genus are small and thread-like, unlike remaining arenicolids which have robust bodies.


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