390 



ZOOLOGY. 



''-.^L/r 



m 



corresponding to the numerous branchial sUts in the other 

 Ascidians, but in Appendicularia each oral 

 aperture leads into a funnel-shaped atrial 

 canal, the open end of which terminates 

 beside the rectum. 



The heart is a large pulsatile ss^c 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 Vertebrate 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), sweUing 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- 

 ing the whole body, and allowing the creature 

 the freest motion within its cavity. 



The general structure of an Ascidian may 

 perhaps be more readily comprehended by a 

 study of a compound Ascidian (AmarcBcium), 

 which grows in white or flesh-colored masses 

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

 rmcium 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. 386'. The mouth leads by the capacious bran- 



Fig. 38U». — Struc- 

 ture of a compound 

 Ascidian, Amarce- 

 cium. A, brancliial 

 sac ; niy Btomacli ; k, 

 intestine ; c, mouth ; 

 c/, testis ; r r'. effer- 

 ent duct or the tes- 

 tis ; C, ovary ; p>, 

 egg in the bouy-cav- 

 Ity; p", eggs in the 

 atrium ; n, anus ; o, 

 shows the site of the 

 heart ; £, liver ; «, 

 openin'js in wal's of 

 branchial chamber. 

 —From Macalis er. 



