812 



NATURAL HISTORY. 



Fig. 245. Body of 

 the Barnacle. 



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, etc. 



534. The class of Annelida, the Worm 

 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 

 (dor sum, 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- 

 cies, 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. 313). Th<^se animals live in 



tubes, attached in groups to stones, shells, and other 



