VARIOUS 249 



very well, and some are entirely pelagic, but most are 

 to be found at the bottom at moderate depths, and 

 some abysmal forms have been found so deep as 

 2,000 fathoms. 



If we look at the stones brought up by the dredge, 

 we are sure to find many of them covered with a fine 

 lacelike incrustation, which, upon close examination, 

 seems to be made up of minute cells, each perforated 

 by a small pore. These cells are formed by the 

 Polyzoa, each cell having a separate inhabitant, which 

 extrudes a tiny crown of tentacles through the opening. 

 Besides the encrusting forms there are other branching 

 ones (Figs. 188, 189), either soft and finely divided, 

 like Hydroids, as already mentioned, or else thick and 

 hard, like small corals. 



Allied to the Polyzoa, but differing from them 

 entirely in appearance, is the group known as the 

 Brachiopods, or lamp shells. The Brachiopods were 

 once classed as Mollusca, on account of their external 

 resemblance to Lamellibranchs, but may generally be 

 distinguished from them by the facts that their valves 

 are unequal in size, and that they are attached by a 

 small stalk, which passes out through a hole in the apex 

 of the larger valve (Fig. 190). They represent a group 

 which was very abundant in early geological times, 

 and of which some forms have persisted almost un- 

 changed to the present day. Many of the species are 

 found at considerable depths. 



The Tunicates, or sea squirts, are generally particu- 

 larly unattractive objects when found among the 

 contents of the dredge. The solitary forms have 

 shapeless sacklike bodies of a gelatinous or leathery 

 texture, often encrusted with mud or sand, and having 

 two openings through which, in the living animal, 

 currents of water enter and leave the body ; the colonial 



