288 CHORD AT A. 



CHAPTER XIX. 

 CHORD AT A. 



ASCIDIA. AMPHIOXUS. 



I. -ASCI DI A. 



Phylum - - Chordata (p. 403). 



Sub-phylum - Atriozoa (p. 404). 



Class - - 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 



Colour and ^^^^^^ ^^ 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 

 which several of the organs are piano-symmetric. 

 Hence, like the Echinodermata, the ascidian has 

 an underlying bilateral or piano-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 Anodouta^ 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. 



