312 NATURAL HISTORY. 
the hull of a ship. In being inclosed 
in a shell it is like the Mollusca, and 
was formerly supposed to belong to 
that sub-kingdom; but the construc- 
tion of the animal itself manifestly 
places it among the Crustaceans. 
533. To this order belong also the 
little Acorn-shells, so called, which are 
found on the sea-shore in abundance 
adhering to rocks, shells, ete. 
534. The class of Annelida, the Worm 
Fig. 24)-—Boly of and Leech tribe, is one of the lower 
classes of the Articulata. The animals 
belonging to it have no articulated members, and there 
is in them a general inferiority of structure. Still, the 
lateral symmetry so characteristic of the Articulata, 
§ 387, is retained in them. The two halves of the body 
are alike. The body is commonly long, slender, and 
more or less cylindrical. The division into segments, 
manifest in most of the Articulata, is in this class more 
manifest internally than externally, it being marked ex- 
ternally only by a wrinkling of the skin. 
535. The class is divided into four orders, which I will 
briefly notice. The first is that of the Dorsi-branchiata 
(dorsum, back, and branchia, gill), having the gills ar- 
ranged in tufts along the length of the body. The ani- 
mals belonging to this order both crawl and swim with 
facility. In tropical climates there are some large spe- 
cles, measuring even four feet, and having the body di- 
vided into four or five hundred segments. The Sea-cen- 
tipede, the Sea-mouse, and the Lob-worm belong to this 
order. 
536. The second order is that of the Tubicola, so 
called because the animals live in tubes. One of the 
most common is the Serpula, one species of which is 
represented in Fig. 246 (p. 318). These animals live in 
shell tubes, attached in groups to stones, shells, and other 
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