288 CHORDATA. 
CHAPTER XIX. 
CHORDATA. 
ASCIDIA. AMPHIOXUS. 
I.—ASCIDIA. 
PHYLUM CHORDATA (p. 403). 
SuB-PHYLUM ATRIOZOA (p. 404). 
Cass TUNICATA (or UROCHORDA) (p. 405). 
Ascidia mentula is a small sac-like marine animal, of 
which common examples may be one inch in length. It 
occurs in great numbers at moderate depths, 
adhering to shells and other foreign bodies, thus 
belonging to the sedentary types. The shape 
is roughly cylindrical and the colour is usually of some dull 
neutral tint. The aboral end is fixed and the oral end 
terminates in a round aperture usually termed the mouth. 
A little way down one side there is another opening called 
the atriopore. 
The plane passing through the two apertures and divid- 
ing the body into equal parts, is the median plane, about 
semitnenty which several of the organs are plano-symmetric. 
* Hence, like the Echinodermata, the ascidian has 
an underlying bilateral or plano-symmetry disguised by a 
more superficial approach to axial symmetry. The surface 
of the body is smooth and devoid of special features. 
If Ascidia be watched in the living condition it can 
be seen that currents of water and food-particles pass into 
the interior by the mouth, whilst a current of water emerges 
by the atriopore. 
As in the cases of Sycandra and Anodonta, the exhalent 
current is devoid of food-particles, which are similarly retained 
for the use of the animal.. 
On being disturbed the living animal can contract its 
body to a considerable extent, and water is then forcibly 
expelled through the mouth. This habit, occurring in indi- 
viduals left dry by the tide, has given rise to the popular 
name of ‘“sea-squirt,” applied to ascidians in general. 
Colour and 
Form. 
