200 



ZOOLOGY. 



corresponding to the numerous branchial slits in the other 

 ^ Ascidians, but in Appendicularia each oral 



^5i/V aperture leads into a funnel-shaped atrial 

 canal, the open end of which terminates 

 beside the rectum. 



The heart is a large pulsatile sac situated 

 between the two lobes of the stomach. The 

 nervous system is much more fully developed 

 than in other Tunicates, and is constructed 

 on the true Vermian type, consisting first of 

 a ganglion situated below the mouth on the 

 side opposite the atrial opening and opposite 

 the anterior end of the endostyle. This 

 nerve-centre throws off nerves to the sides of 

 the mouth, and from it posteriorly extends a 

 long cord past the oesophagus to the base of 

 the tail, thence it extends along one side of 

 the axis of the tail (urochord), swelling at 

 regular intervals into small ganglia, from 

 which from two to five small nerves radiate. 

 On the cephalic ganglion a round ear-vesicle 

 is attached. Behind the posterior turn of 

 the digestive canal is the testis and ovary, 

 the Appendicularia being hermaphrodite, as 

 Fol claims, though the ovary is developed 

 later than the testis. The Appendicularia 

 has no test, but secretes a fibrous envelope, 

 which is at first gelatinous, loosely surround- 

 Fig. 137. struc- ing the whole body, and allowing the creature 



tnre of a compound ,, , . . , . ., ., 



Ascidian, Amarce- the freest motion Wltlllll its Cavity. 



The general structure of an Ascidian may 

 perhaps be more readily comprehended by a 

 study of a compound Ascidian (Amar cerium), 

 egg in the body-cay- w hich grows in white or flesh-colored masses 



ity; p , eggs in the 



on sea-weeds, etc. On removing an Ama- 

 rcecium from the mass and placing it under 

 the microscope, its structure can be per- 

 ceived. The body is long and slender, as 

 seen in Fig. 137. The mouth leads by the capacious bran- 



ovar h - e 



'j : .f i "i 



atrium ; n, anus ; 0, 

 shows the site of the 

 heart ; /, liver ; e, 

 openings in wal ; sof 

 branchial chamber. 

 From Macalis er. 



