ASCIDIA. 61 
neighbouring stigmata, as shown in a small Ascidian under 
the microscope. 
NERVOUS 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 (Pl. II., fig. 4, 2g.) 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 (Pl. I1., fig. 6). In 
Ascidia mentula 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 ganglion 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 (Pl. I1., 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 mentula are mostly 
pyriform or triangular in form, and are bi-polar or multi- 
polar, and finely granular. 
The Ascidian has little papille containing sensory cells 
In its ectoderm, especially round the apertures, and has 
