480 



TUNICATA. 



Fig. 243. 



as food are taken in, while the other (/) gives egress to the ova and 

 excrementitious matter. The soft outer covering is permeated by blood- 

 vessels, which ramify extensively in it ; it is moreover covered externally 

 with an epidermic layer, and lined 

 within by a serous vascular mem- 

 brane, which, in the neighbourhood 

 of the two orifices, is reflected from 

 it on the body of the animal lodged 

 inside. The creature hangs loosely 

 in its outer covering, to which it is 

 only connected at the two apertures 

 by means of the reflexion of the peri- 

 toneal membrane above mentioned. 



(1261.) On removing a portion of 

 the exterior tunic, that in reality re- 

 presents the shells of a bivalve Mol- 

 lusk, the soft parts of the Ascidian 

 are displayed. The body is seen to 

 be covered with a muscular invest- 

 ment (the mantle) (fig. 243, b b, c), 

 composed of longitudinal, circular, 

 and oblique fibres, which cross each 

 other in various directions, so as to 

 compress by their contraction the 

 viscera contained within, and this 

 so forcibly that, when alarmed, the 

 animal can expel the water from its 

 branchial sac, immediately to be de- 

 scribed, in a thin continuous stream, 

 sometimes projected to a distance of 

 many inches. 



(1262.) Respiration is effected in 

 an apparatus of very peculiar con- 

 trivance, to the examination of 

 which we must now request the at- 

 tention of the student. A consider- 

 able portion of the interior of the 

 body is occupied by a circumscribed 

 cavity, that opens externally by the 

 orifice h-, into this bag a bristle 

 has been introduced in the dissec- 

 tion, represented in the figure (fig. 243) ; its walls are seen to be com- 

 posed of a thin but very vascular membrane (d d d), that has been 

 partially turned back, so as to display the interior of the respiratory 

 sac. The membrane (fig. 243, d d d; fig. 244, e), when examined 



Structure of Phallusia nigra : a a a, ex- 

 ternal envelope ; b b, the mantle ; c, mantle 

 reflected so as to display d d, the membrane 

 lining the respiratory sac ; e e, alimentary 

 canal ; f, excretory orifice ; g, orifice of ovi- 

 duct ; h, oral aperture ; /, a piece of coral to 

 which the animal is fixed; n, the anus. 

 (After Hunter.) 



