42 THE VOYAGE OF H.M.S. CHALLENGER. 



animal. It lies in the mantle near its inner surface, underneath the muscular la) T ers, 

 and above the neural gland. 



It has usually an oblong form, slightly constricted in the middle, and giving off nerve 

 trunks from the expanded ends, one of which is directed anteriorly and the other 

 posteriorly. 



In the Ascidias Simplices, where the branchial and atrial apertures are usually approxi- 

 mated, the nerve ganglion lies in the angle between them, almost invariably nearer the 

 branchial than the atrial aperture (the reverse is the case in Ascidia compressa and 

 Asctdia fusiformis), and the anteriorly and posteriorly directed nerve trunks run directly 

 to the neighbourhood of the apertures, and give off branches to the tentacles, the ocelli, 

 and the margins of the lobes. 



In Appendicidaria there is a large ganglion, with three dilatations, placed in the 

 usual position on the dorsal side of the anterior end of the pharynx, and giving off, in 

 addition to nerves to the branchial aperture, a long nerve trunk, which runs back along- 

 side the oesophagus and past the stomach till it reaches the base of the caudal appendage. 

 Here it goes to the left side of the urochord, along which it runs to the extremity, giving 

 off nerves from ganglionic enlargements which occur at intervals. The most anterior of 

 these caudal ganglia is the largest. Fol states that this gangliated nerve trunk is tra- 

 versed by a fine canal. 



The periphery of the ganglion is formed of globular or pyriform unipolar nerve cells, 

 while the centre is a mass of nerve fibres, with a very few small nerve cells scattered 

 through it. The nerve trunks arise from this central fibrous part. 



Sense Organs. 



In the adult Simple and Compound Ascidians the only structures which can be called 

 sense organs — now that the sensory nature of the so-called " olfactory tubercle " has 

 been disproved — are the tentacles round the base of the branchial siphon, which have 

 probably some sensory function, and the ocelli or pigment spots situated around the 

 branchial and atrial apertures. They are usually equal in number to the lobes surround- 

 ing the apertures, and are situated in the clefts between them. The pigment, which is 

 almost invariably red, is imbedded in the mantle just above the anterior edge of the 

 sphincter muscle, where the test is usually thin and transparent, and is supplied by 

 branches of the nerve trunks from the anterior end of the nerve ganglion. 



In Appendicularia there is a rounded otocyst, containing a spherical otolith, attached 

 to the main ganglion, while Fol has described a number of fine tactile setaa situated 

 around the branchial aperture. In some forms of Doliohtm there is also a spherical 

 otocyst, containing a single otolith and supplied by a nerve from the ganglion. 



