390 



ZOOLOGY. 





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corresponding to the numerous branchial shts in the other 

 Ascidians, but in Appendicularia each orai 

 aperture leads into a funnel-shaped atrial 

 canal, the open end of which terminates 

 beside the rectum. 



The heart is a large pulsatile s^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 motith 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 cephahc ganglion a round ear- vesicle 

 is attached. Behind the posterior turn of 

 the digestive canal is the testis and ovaiy, 

 the Appendicularia being hermaphrodite, as 

 I'ol 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 {Amarcechcm), 

 which grows in white or flesh-colored masses 

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

 rcecium from the mass and placing it under 

 the microscope, its structure can be per- 

 ceived. The body is long and slender, as 

 386'. The mouth leads by the capacious bran- 



Fig. 38C'. — Struc- 

 tnre of a compcwnd 

 Ascidian, Amarce- 

 cium. A, branchial 

 sac ; m, stomach ; ^, 

 intestine ; c, mouth ; 

 (/, testis ; r r^, effer- 

 ent duct of the tes- 

 tis ; C, ovary ; p'^ 

 egg in the body-cav- 

 ityj p", egijs in the 

 atnam ; », anus ; o, 

 shows the site of the 

 heart ; t, liver ; e, 

 openings in walls of 

 branchial chamber. 

 —Prom Macalisier. 



seen in Fis 



