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Order POLYCH.ETA. 



Marine worms usually with a definite head (prostomium), which dorsally bears eyes 

 and tentacles, and ventrally palps, besides possessing other sense-organs. 



From the buccal region (peristomium), which frequently carries cirri or other organs, 

 the fore-gut often sends out a protrusible proboscis — armed or unarmed. (Esophagus may 

 have a pair of diverticula. Alimentary canal simple or branched, and a median dorsal 

 and median ventral mesentery may be present. 



Body of one or more regions, generally elongate and cylindrical, sometimes flattened 

 and compressed, of numerous segments, which internally are marked rather by the 

 oblique muscles than by diaphragms. The last segment in many has long ventral (anal) 

 cirri. It is invested by a thin, tough cuticle covering a granular layer, 1 bounded 

 internally by basement-lining, then a more or less complete layer of circular muscular 

 fibres, while within are four longitudinal muscular bands, — as a rule, two dorsal and 

 two ventral. The oblique muscles converge from the dorso-lateral region to the vicinity 

 of the nerve-cords. 



The segments in the free examples bear lateral processes of the body- wall (feet or 

 parapodia), and each foot is frequently divided into a dorsal and a ventral lobe, with a 

 spine (aciculum) to which motor muscles are attached, and a group of specially 

 differentiated bristles, besides a dorsal and a ventral cirrus. 



Respiratory organs either free branchiae or the general surface. 



Circulatory organs generally closed, contractile or non-contractile vessels. Blood 

 red, pink, green, or pale, corpusculated or non-corpusculated (?) ; occasionally absent. 



Perivisceral (coelomic) fluid abundant, highly organised and corpusculated. In 

 certain forms the perivisceral corpuscles are coloured, and the whole may act as a 

 substitute for the circulatory fluid. 



Nervous system consists of cephalic and subcesophageal ganglia with commissures, 

 separate or closely applied longitudinal ganglionated trunks, with or without neural 

 canals ; placed in or internal to the granular layer under the cuticle. A stomato- 

 gastric system of nerves also occurs. 



Reproductive organs are developments of the perivisceral (coelomic) epithelium in 

 connection with the ventral blood-vessels. The products escape by the segmental 

 organs (nephridia), or by rupture of the body- wall. Sexes, as a rule, are separate, but 

 some are hermaphrodite. Polymorphism and asexual reproduction also occur. 



The segmental organs may act as genital ducts, and are arranged in pairs in few or 

 many segments. 



Larva a trochophore. Metamorphosis during development. 



Free or sedentary (errant or tubicolous). 



1 Formerly named hypoderm, a term, however, which has given rise to misapprehensions. 



