18 INVERTEBRATE ANIMA.LS. 



a chain ; and allies. Polyzoarium chiefly articulated 

 and flexible. 



II Examples of Cellularia, Salicornaria, Bicellaria, 

 Emma, Bugula, &c. 



Upper Compartment. 

 Diagram of a Polyzoon, and of an " avicularium," 

 or Bird's-bead process. Coloured drawings of 

 fresh-water Polyzoa, after Allman. 

 The animals of the Polyzoa are always minute, 

 and live associated in colonies. They are 

 much in advance of the Ccelenterata in com- 

 plexity of structure, the food canal being 

 entirely shut off from the body-cavity, and the 

 nervous system being well defined. 



Group 52.— Class TUNICATA. Tunica, a cloak. The 

 Ascidians, or Tunicaries, are marine animals, having 

 their internal organs protected by a double integu- 

 ment, tough and elastic, in place of a shell. Estimated 

 number of species : recent, 150 ; British, 73. 



If Coloured drawings of solitary, social, and com- 

 pound Ascidians. 



Upper Compartment. 



Preparations in spirits of Salpians in the chained 

 and solitary states ; also of Pyrosoma, and 

 many other Ascidians. Diagrams of the early 

 development of an Ascidian, shewing by the 

 presence of a pseudo-notochord its approach 

 to the form of the vertebrate embryo. 



In a few cases it has been shewn that the Ascidians 

 are degenerated forms of vertebrate animals ; 

 and in some recent systems of classification 



