424 GENERAL STRUCTURE OF TCNICATA. 



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 comes 

 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 par- 

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



970. Within the external tunic is a muscular coat, repre- 

 senting the mantle^ consisting of fibres crossing one another in 

 various directions, by which compression may be exercised on 

 the contents of the cavity it surrounds. The openings which 

 have been mentioned as existing in the outer coat, penetrate this 

 one also ; and they are guarded by muscular fibres, arranged in 

 a circular manner around them ; by the action of which rings, 

 they may be entirely closed. 



971. The cavity of the mantle is principally occupied by a 

 large bag, composed of a delicate membrane, over which the 

 blood-vessels are minutely distributed ; this evidently replaces 

 the gills of higher Mollusca, and it is accordingly termed the 

 branchial sac. It is closely adherent to the mantle around one 

 of the orifices, which leads directly into it, and which is there- 

 fore called the branchial orifice (a, Fig. 601). It also receives 

 support from it elsewhere ; but it by no means occupies the 

 whole of the cavity. There is a considerable space at the 

 bottom of it, which is occupied by the digestive and reproductive 

 viscera ; and there is also a space in the neighbourhood of the 

 second orifice (b), which serves for the discharge of several dif- 

 ferent egesta* and may thence be termed the funnel or vent. 

 The branchial sac is probably to be regarded as formed by a 

 dilatation of the pharynx, or funnel-shaped commencement of 

 the oesophagus; and the branchial orifice as the real mouth. 

 We shall hereafter see, that a very near approach to this struc- 

 ture is presented by certain Polypes (Fig. 624). 



972. The entrance to the digestive tube, lies at the bottom 

 of the branchial sac ; and the alimentary particles are derived 



* Fluid or solid matter to be carried out of the system. 



