Chap. III.] 



BRA CHIOPODA : BR } r ozoA . 



101 



unequal valves which make up their shell, and by the 

 characters of their nervous system. The so-called 

 arms (Fig. 49 ; 6) are outgrowths of the prse-oral disc 

 of the larva, at the edges of which the tentacles or 

 cirri are set. This great development of their arms is 

 to be correlated with the fixed habit of the adult. 



Fig. 50. Bugula purpurotincta. Nat. size. (After Hiucks.) 



Terebratula and Lingula (which is stalked) are ex- 

 amples of this isolated and geologically ancient group. 



2. The Bryozoa have likewise been placed with 

 the Mollusca ; they are clearly degenerate forms 

 which, by the characters of their larvse, appear to 

 have been descended from an ancestor common to 

 them and the Chaetopoda. Balfour has suggested 

 that they become fixed by their prse-oral lobe. They 

 live in colonies, and are the forms that are popularly 

 known as sea-mats or sea-mosses (Fig. 50). 



3. The Chsetognatlia (as represented by Sagitta) 

 are forms that have relations to the Chastopoda and 



