TUNICATA 663 



CLASS 3. ASCIDIACEA.* 



The aseidians. Sac-shaped, mostly sessile and either simple or com- 

 pound tunicates with oral and cloacal openings usually placed near to- 

 gether, each at the end of a contractile projection called the siphon (Fig. 

 998). The cellulose tunic is usually thick and tough, but in some forms 

 it is gelatinous or transparent, and is often rough and warty on the 

 outside. Both openings can be closed by sphincter muscles and are often 

 provided with a number of sensory lobes bearing in some cases pigment 

 spots. The branchial sac (8) is very large and its entire wall is pierced 

 by more or less regular series of slits or stigmata, giving it a lattice-like 

 appearance. Its upper margin is surrounded by a circle of tentacles, 

 which thus lie just back of the mouth. Endostyle (6), peribranchial 

 band (5), and dorsal lamina (9) are present. On each side of the 

 branchial sac is the peribranchial space, into which the respiratory water 

 as well as the discharges from the genital and digestive organs are 

 poured, and which communicates with the outside through the cloacal 

 opening (Fig. 999,10). The oesophagus, stomach, and the short intestine 

 are either at the side of or beneath the branchial sac. The anus opens 

 into the peribranchial chamber near the cloaca or into the cloaca itself. 

 The principal ganglion (4) is situated between the two siphons in the 

 mantle and adjacent to it is the subneural gland (3), which is supposed 

 to be homologous to the hypophysis of vertebrates. A duct joins this 

 gland with the pharynx, the opening being at the end of a projection 

 called the dorsal tubercle. A ductless kidney or scattered renal cells in 

 the bend of the intestine contain uric acid crystals. 



Aseidians are hermaphroditic, the gonads lying close together. From 

 the egg is hatched a long-tailed larva which has the appearance and gen- 

 eral structure of an appendicularian. This larva attaches itself to some 

 fixed object by three papillae at the forward end, and a complex meta- 

 morphosis proceeds, during which the tail is absorbed, the peribranchial 

 chamber develops, and the body assumes the spherical or cylindrical shape 

 of the adult. 



Aseidians are either simple or colonial and with the exception of the 

 Pyrosomidae are all sessile, being attached usually to rocks or seaweed. 

 The colonial forms arise by a process of budding from one another and 

 form incrusting or erect masses on shells, seaweed, etc. Several species 

 are used for food, large numbers being brought into the fish markets of 



* See "Some Ascidlans from Puget Sound," by W. E. Rltter, Ann. N. Y. Acad. Scl., 

 Vol. 12, p. 589, 1909. "The Aseidians of Bermuda," by W. G. Van Name, Trans. 

 Conn. Acad., Vol. 11, p. 325, 1902. "Mosaic Development in Ascidian Eggs," by 

 E. G. Conklin, Jour. Ex. Zool., II, p. 145, 1905. "The Simple Aseidians from the 

 North Eastern Pacific," etc., by W. E. Ritter, Proc. U. S. Nat. Mus., Vol. 45. 

 p. 427, 1913. 



