THE MICROSCOPE IN ZOOLOGY. 169 



corals, either membranous or calcareous, made up of minute 

 cabin-like cells, which are either thin crusts on sea-weeds, 

 rocks, etc., or slender moss-like tufts, or groups of thin 

 curving plates, or net-like fronds, and sometimes thread- 

 like lines or open reticulations. The cells of a group have 

 no connection with a common tube, as the Hydroids, but 

 the alimentary system of each little Bryozoon is indepen- 

 dent. 



Many of the Polyzoa have curious appendages to their 

 cells, of two kinds ; the first are called birds'-head pro- 

 cesses or avicularia. They consist of a body, a hinge or 

 lower jaw-like process, and a stalk. The lower portion is 

 moved by an elevator and depressor muscle, and during 

 life the motion is constant. The second kind, or vibracula, 

 is a hollow process from which vibratile filaments project 

 (Plate XY, Figs. 127, 128). 



X. TUNICATA. These molluscs are so named from the 

 leathery or cartilaginous tunic which envelops them, and 

 which often contains calcareous spicula. Like the Bryo- 

 zoa they tend to produce composite structures by gemma- 

 tion, but they have no ciliated tentacles. They are of 

 most interest to the microscopist from the peculiar actions 

 of their respiratory and circulatory organs, which may be 

 seen through the transparent walls of small specimens. 

 The branchial or respiratory sac has a beautiful network 

 of bloodvessels, and is studded with vibratile cilia for dif- 

 fusing water over the membrane. The circulation is re- 

 markable from the alternation of its direction. 



The smaller Tunicata are usually found aggregate, in- 

 vesting rocks, stones and shells, or sea-weeds ; a few are 

 free. 



Synopsis of the Families. 



A. Attached ; mantle and test united only at the ori- 

 fices. 



1. Botryllidce. Bodies united into systems. 



