GENERAL STRUCTURE OF TUNICATA. 425 



bunches of grapes. In the larger species, additional firmness is 

 often given to this tunic, by the agglutination of particles of sand, 

 bits of gravel- or shell, or other substances with which it conies 

 in contact, to its exterior ; and a complete envelope is sometimes 

 formed in this manner, which might be compared to the shells 

 of the higher Mollusca, but that it is altogether derived from 

 external sources, the glutinous matter which unites the parti- 

 cles together, being the only part furnished by the animal itself. 



1055. Within the external tunic is a second coat, or inner 

 tunic, which is continuous with the outer coat at the apertures, 

 but is separated from it by a considerable space throughout the 

 rest of the body. The inner tunic is furnished with muscular 

 bands, which vary in number and direction, by which compres- 

 sion may be exercised upon the contents of the cavity which it 

 surrounds. The space between the two tunics is filled up with 

 a system of sinuses, serving for the circulation of the blood ; 

 and the cavity enclosed by the inner tunic, which is open at 

 both ends, is constantly filled with water, which thus comes in 

 sufficient contact with the blood to effect the function of respir- 

 ation. This inner cavity is consequently to be regarded as a 

 respiratory sac. The function of respiration is performed in 

 part through the membrane of the inner tunic itself; but there is 

 usually a further provision for this purpose in the form either of 

 a band, running from end to end of the respiratory cavity, or of 

 a network of filaments attached to the inner tunic ; both these 

 forms of respiratory organs are clothed with cilia. 



1056. The entrance to the digestive tube lies at the posterior 

 portion of the respiratory sac ; and the alimentary particles are 

 derived from the water introduced into the latter for the purpose 

 of respiration. The resophagus is short, and leads to a capacious 

 stomach surrounded by clusters of biliary follicles (ANIM. PHYS. 

 356), the rudimentary form of a liver. The intestine generally 

 makes one or two turns in the space between the branchial sac 

 and the mantle ; and terminates in the neighbourhood of the 

 posterior funnel. The ovaria are usually large, and lie amongst 

 the viscera ; their excretory duct also terminates in the same 

 situation. The posterior funnel thus serves to carry out of the 



