TETHYODEA. 



95 



into a long nerve, which at the base of the tail dilates to a ganglion, 

 and in its further course forms a number of smaller ganglia (fig. 

 563). The reduction of the central nervous system to the simple 

 ganglion of the Ascidian begins after loss of the tail, and after 

 development of the branchial basket. 



Of sense organs the processes of the integument which serve for 

 tactile purposes (the lobes of the oral and atrial apertures and the 

 tentacles), and peripheral nerves, ending in epithelial cells, are most 

 widely distributed. The large ciliated cells on the edge of the 

 mouth of the Copelata must be placed in the same category. The 

 ciliated pit is to be regarded as an olfactory organ. It consists of a 

 depression in the wall ( Tz 



of the pharynx lined 

 with ciliated cells, and 

 is situated in front of 

 the ganglion. Accord- 

 ing to Julin, it is, to- 

 gether with a gland 

 situated beneath the 

 ganglion, to be regarded 

 as the equivalent of 

 the hypophysis. In the 

 Copelata the ciliated pit 

 is elongated and lies on 

 the right side of the 

 ganglion. 



There is an auditory 

 vesicle on the left side 

 of the ganglion in the 

 Copelata. This structure, which is developed from a cell of the wall 

 of the ganglion, is found in the Ascidian larvae, but degenerates soon 

 after the attachment of the larva. Paired auditory vesicles appear 

 in Pyrosoma where they are connected with the ganglion by a short 

 stalk. 



Masses of pigment which are present, with great regularity on the 

 lips of the large openings of the body in the simple and compound 

 Ascidians may be interpreted as eye spots. The eye of the Ascidian 

 larvse, which lies on the ganglion and originates from a part of the 

 neural canal, has a more complicated structure. Later it degenerates, 

 but in Pyrosoma it is retained in the adult condition and possesses a 

 lens-like structure. 



FIG. 563. Nervous system of Appendicularia (Fritillaria 

 furcata (after Fol.). G, Ganglion; N, body nerve; N, 

 lateral nerve ; Ot, otolifch vesicle ; Rg, olfactory pit ; Tz, 

 tactile cells with their nerve ; Wb, arch of cilia. 



