TUNICATA. 



369 



shapeless beings ; and the anatomist is surprised to find how re- 

 markably the beauty and delicacy pig. 174t 

 of their interior contrasts with 

 their rude external appearance. 

 In the species selected for special 

 description (Phallusia mgra),the 

 external envelope (jig. 174, a, 

 a, a) is soft and gelatinous in its 

 texture, fixed at its base to a 

 piece of coral (/), and exhibiting 

 at its opposite extremity two ori- 

 fices (A, y), placed upon pro- 

 minent portions of the body. 

 Through the most elevated of 

 these orifices (h) the water re- 

 quired for respiration, and the 

 materials used as food, are taken 

 in ; while the other (f) gives 

 egress to the ova and excremen- 

 titious matter. The soft outer 

 covering is permeated by blood- 

 vessels which ramify extensively 

 in it ; it is moreover covered ex- 

 ternally with an epidermic layer, 

 and lined within by a serous vas- 

 cular membrane, which, in the 

 neighbourhood of the two orifices, 

 is reflected from it on to 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 reflection of the peritoneal membrane above men- 

 tioned. 



(402.) On removing a portion of the exterior tunic, that in 

 reality represents the shells of a bivalve Mollusk, the soft parts of 

 the Ascidian are displayed. The body is seen to be covered with a 

 muscular investment (the mantle) (Jig. 174, 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 



