25 



neighbouring stigmata, as shown in a small Ascidian under 

 the microscope. 



Neevous System and Sense Organs. 



The nervous system of the Ascidian consists of a single 

 elongated Ganglion or "brain," placed near the front end 

 of the body, in the dorsal median line, between the 

 branchial and atrial apertures (PI. II., fig. 4, n.g.) It 

 gives off several large nerves at each end, which break up 

 into the fibres that go to the different parts of the body, 

 and especially to the lobes and the muscles surrounding 

 the branchial and atrial apertures (PI. II., fig. (J). In 

 Ascidia men tula there are four chief nerves from the 

 anterior end of the ganglion, one to the surface of the 

 body and the other three to the branchial siphon ; while 

 three nerves run from the posterior end to the atrial 

 aperture and wall of cloaca. There are also one or two 

 smaller lateral nerves that leave the sides of the ganglion. 



This jranolion is the degenerate remains of the anterior 

 part of the cerebro-spinal nervous system of the tailed 

 larval Ascidian. The posterior or spinal part has almost 

 entirely disappeared in most adult Tunicata ; but its 

 remains may be traced in a tract of degenerate nerve 

 tissue (the dorsal nerve cord) which runs posteriorly from 

 the ganglion (PI. II., fig. 6, d.n.) above the base of the 

 dorsal lamina towards the viscera. The ganglion has 

 small rounded nerve cells on its surface, while the centre 

 is a mass of interlacing nerve fibres. Small cells are 

 also found scattered along the course of the dorsal nerve 

 cord. The nerve cells in Ascidia vieiitula are mostly 

 pyriform or triangular in form, and are bi-polar or multi- 

 polar, and finely granular. 



The Ascidian has little papillsB containing sensory cells 

 in its ectoderm, especially round the apertures, and has 



