192 MARINE INVERTEBRATES 



group. They are plentiful everywhere, occurring between tide- 

 marks and at great depths on the floor of the sea. Rocks pro- 

 tected from the sun are often incrusted with the calcareous forms, 

 while branching species hang from the rocks and adorn rock 

 pools. Membrcmipora, in lace-like sheets, will be found on the 

 larger seaweeds as well as on various submerged objects. The 

 beautiful little Crisia may be looked for on the fronds of red 

 algas, and Alcyonidium, in soft moss-like patches, on Fucus. On 

 some beaches are scattered the sea-mats, Flustra foliacea, looking 

 like bits of brown paper. A pocket-lens will reveal the marvel- 

 ous and orderly arrangement of the two layers of cells, or zooecia, 

 placed back to back. There is not a more surprising object in 

 the animal kingdom than a living branch of Bugula. The cells 

 on its spiral, profusely branched clusters have appendages re- 

 sembling a bird's head, with its bill constantly opening and shut- 

 ting. Of the erect polyzoans some are entirely calcareous and 

 rigid; such species live in deep water, and their colonies are 

 called corallines, as are also the calcareous algae. They have ex- 

 isted in vast numbers since early geologic times, and are plentiful 

 in the Tertiary deposits, known as Coralline Crags. Other erect 

 forms have a calcareous framework, which is not continuous, but 

 has horny intervals, so that the colonies are flexible and can be 

 swayed by the tides in shallow water where they live. Others 

 have complete horny exoskeletons. The same differences occur 

 in the incrusting forms, those on the seaweeds being either flex- 

 ible, so that they bend with the plant, or if entirely calcareous 

 are so small that they do not break when the plant sways to and 

 fro, Still others are gelatinous and resemble bits of sponge. 

 The calcareous forms are usually ornamented with ridges which 

 form beautiful patterns. 



The individuals of the polyzoan colonies are small polyp-like 

 organisms which have a cup-like inclosure of horny or of calca- 

 reous substance, or of the two combined, as mentioned above. 

 This inclosing sheath is the exterior or cuticle of the animal and 

 is called the zowciiim, the whole colony being called the zoarium. 

 The animal substance within the zooecium is the polypide, and 

 consists of the mouth, tentacles, and alimentary system. A disk 

 at the anterior end of the body, bearing the circle of tentacles, is 



