in. 4 MOVEMENTS OF ASCIDIANS 65 



the pharynx. The nervous system shows little sign of those continuous 

 activities that produce the varied and 'spontaneous' acts of behaviour 

 in higher forms. Nevertheless, it would be unwise to suppose that the 

 nerves are only activated by external stimuli. There are some sugges- 

 tions that even in these simple animals rhythmical activities are 

 initiated from within. The food-collecting operations of the pharyn- 

 geal wall involve rhythmical movement of the papillae by their 

 muscles. Further, in many species of ascidians there are regular 

 contractions of the siphons and body musculature in rotation, with 



Fig. 35. Rhythmical 'spontaneous' contractions of Styela shown by attaching levers 



to the two siphons. Branchial siphon above, atrial siphon below. The time-marker 



shows intervals of 5 minutes. (From Yamaguchi.) 



a frequency of 8-27 per hour (Fig. 35). These contractions are 

 especially marked when the animal is in filtered water and they may 

 be some form of 'hunger' contraction, directed towards the obtaining 

 of food. More water is moved by these contractions than by the ciliary 

 current. Their presence is a striking warning of the dangers of assum- 

 ing that even the simplest nervous system operates only when 

 stimulated from outside. 



The neural gland is a sac lying beneath the ganglion and opening 

 by a ciliated funnel on the roof of the pharynx. It arises mainly from 

 the ectoderm of the larval nervous system, in part from the pharynx. 

 This double embryological origin, and its position, suggest that the 

 neural gland may be compared with the infundibulum and hypophysis 

 of vertebrates. There is an obvious similarity with Hatschek's pit of 

 amphioxus. Both seem to be receptor organs, testing the water stream 

 and also producing mucus. The subneural gland has also been held 

 to have a similarity to the pituitary in that it controls the release of 

 gametes. When eggs or sperms of the same species are present in the 

 water, signals from the neural gland apparently produce discharge 

 from the gonad. The pathway of the signals is said to be partly hor- 

 monal, partly nervous. Discharge is produced by injection of extract of 



