OR OSSEOUS SYSTEM. 43 



skeleton is most dense, thick, and opaque in the larger 

 isolated forms of ascidise, and is most soft, delicate and 

 transparent in the aggregated or compound forms of tuni- 

 cata. Two very dissimilar kinds of tunicated animals are 

 represented in Fig. 19, where 1, is the cynthia papillata, 

 and 2, the pyrosoma gigantium ; the first is permanently 

 fixed, like most of the animals of this class, FIG. 19. 

 and the second is an aggregate mass of nu- 

 merous individuals which floats by their com- 

 bined movements freely through the sea. In 

 the cynthia we observe the transparent tough 

 cartilaginous tunic forming a sac, which is 

 entirely closed except at the respiratory orifice 

 (,) which admits the currents of water brought 

 by the vibratile cilia of the gills, and the 

 vent (,) by which the currents escape from 

 the respiratory cavity. The lining muscular 

 tunic of the mantle is most firmly united to the 

 sac around these two orifices, where there are 

 also distinct sphincter muscles. At the bottom of the sac 

 is seen the stomach (c,) the great branchial vein (d,) which 

 dilates below the stomach to form a heart, the convoluted 

 intestine (e,) the ovary (f y ) and the liver (#,) enveloping the 

 pyloric end of the stomach. The pyrosoma, Fig. 19. 2, is 

 a long tube closed at the upper end (a,) open at the lower 

 extremity (#,) and composed of innumerable distinct in- 

 dividuals (c, ,) which are similarly organized internally to 

 the cynthia, but have their respiratory orifices on the 

 sides of the long projecting external papillae (c, c,) and the 

 vents or anal orifices of all the separate individuals open- 

 ing into the interior of the tube ; so that by the act of 

 respiration alone, this transparent and luminous tube is 

 carried up through the still seas. The exterior tunics of 

 these component animals are thin, soft, transparent, and 

 of a bluish white colour, and are the only parts by which 

 they appear to be united together. In some of the com- 

 pound forms of these tunicata, however, the enveloping 

 tunic has a third opening, which admits the currents of 

 respiration or circulation to extend from one individual to 

 another. 



XV. Conchifera. The shells of these animals consist 



