212 ANNELIDA. 



(256.) Dorsibranchiata. We have gone too minutely into the 

 anatomy of the two preceding orders of Annelidans to render an 

 equally detailed account of the structure of the Dorsibranchiata ne- 

 cessary ; we must therefore restrict our observations to those points 

 in which remarkable variations from what has already been described 

 present themselves to our notice. These worms are all inhabit- 

 ants of the sea ; and although upon our own coasts they seldom 

 attain to very considerable dimensions, rarely exceeding a few 

 inches in length, in tropical climates some species are found of 

 comparatively gigantic proportions, having their bodies composed 

 of four or five hundred segments, and occasionally measuring four 

 feet from one end to the other. 



We have already seen ( 233) that, in the more perfectly or- 

 ganized forms of these worms, each segment of the body supports 

 certain external, moveable appendages adapted to assist in locomo- 

 tion, which are usually called the feet, or more properly the oars ; 

 they present great diversity of appearance, and, from the nature and 

 arrangement of the different parts composing them, are of material 

 assistance to the systematic zoologist, as they afford important 

 characters for the establishment of generic and specific differences. 

 In the section of Leodicea antennata already given, (Jig. 77, 2,) 

 these parts are seen in a very intelligible form, and are visibly 

 composed of three distinct structures adapted to different uses. 

 The first, which occupies the uppermost position, is the respiratory 

 apparatus (b) ; in Leodicea its structure is extremely simple, 

 being composed of a central stem from which a single series of 

 vascular filaments is sent off, giving the organ a pectinated ap- 

 pearance ; but in other cases the branchial tuft is far more con- 

 siderably developed, dividing and subdividing into minute ramifi- 

 cations, and thus offering a more considerable surface to the 

 surrounding element. In most instances, as in Leodicea (Jig. 

 77, 1), these respiratory arbuscles are placed along the entire 

 length of the body, being appended to every segment, with the 

 exception perhaps of a few of the most anterior ; nevertheless, in 

 some species, their distribution is more partial, and their presence 

 is restricted to a few rings of the animal. 



In Arenicola piscatorum, for instance, (Jig. 87,) a worm 

 met with abundantly upon our own coasts, and eagerly sought 

 after as a bait by fishermen, who dig it from the holes which 

 it excavates in the sand, the branchiae (b) are confined to the 

 central portion of the body, where they form on each side a series 



