PRIMITIVE VERTEBRATES 



297 



but not extending so far forwards. There is, however, no distinct 

 brain. 



It is usual to find this animal buried in the sand in a vertical 

 position, with the head end projecting; but it can also swim, and 

 is able to burrow in the sand with great rapidity. 



Sub-class II. — SEA-SQUIRTS (Urochorda) 



Among the objects cast up by the tide on the sea-shore, or 

 found attached to rocks which are uncovered at low -water, are 

 certain leathery-looking objects which when touched emit a jet 

 of water, a habit which has 

 earned for them the name of Sea- 

 Squirts. On account of the firm 

 cover or tunic with which they 

 are invested the name of Tuni- 

 cates has also been widely used. 

 They are the first fixed or seden- 

 tary animals with which we have 

 had to deal, and this mode of life 

 has had a profound influence 

 upon their structure. A com- 

 mon British species, Ascidia 

 -mentula, may be taken as an 

 example. 



The plump rounded body is 

 attached by one end to some 

 foreign object, while at the other 

 end may be seen two orifices 

 (see p. 296, fig. 171), each 

 placed on a projection, so as to 

 give a distant resemblance to 

 the skin bottles used in the East, 

 and which is embodied in the name of Ascidians (Gk. askos, a 

 wine-skin ; eidos, like) often applied to these forms. One of the 

 openings is situated at the extreme end and the other somewhat 

 on one side. The former is the mouth and the latter the atriopore, 

 and observation of a living specimen placed under water will 

 show that currents set into one and out of the other. The 

 protective tunic or test, which is thick and gristly in texture. 



Fig. 172. — Diagram to explain structure of a simple 

 Ascidian. The animal is seen from right side with dorsal 

 surface to left and front end above. The arrows indi- 

 cate course of water- currents which take food and oxygen 

 into mouth, pass through perforations in pharynx into 

 atrial cavity, and carrj' waste products to exterior 

 through atriopore. 



