FISHES AND SEA-SQUIRTS. 



295 



FIG. 84. Polycarpa rustica, a common 

 sea-squirt. 



distinctions from Polycarpa rustica, another common red 

 Ascidian, which has four folds in the branchial sac at each 

 side, and has reproductive 

 organs on both sides of 

 the body-wall. 



The " gooseberry " is 

 very common, and from 

 its tough, leathery coat it 

 is easy to cut into sec- 

 tions, which show the 

 anatomy clearly. By 

 taking a pair of scissors, 

 and clipping a few speci- 

 mens up at different angles 

 and in various planes, the 

 structure can be more 

 readily understood than 

 by even a very careful dissection of a soft form like dona. 

 As to the humanitarian aspect of the matter, it is difficult 

 to think that one can have more scruples than about slicing 

 a cabbage, but a tender conscience may be appeased by 

 immersing the specimens for a short time in methylated 

 spirit, and this will also assist the subsequent examination 

 by hardening the tissues. 



Besides dona intestinalis, which we have already described, 

 several species of the genus Ascidiella are common on the 

 rocks. It is hardly possible to describe the specific characters 

 without going into details which are a little beyond our 

 reach, but we may note that a form called A. virginea is 

 very abundant in the Firth of Forth, where it grows 

 socially in dense masses attached to seaweed, or Polyzoa, 

 and is cast on the beach after every storm. It has a 

 delicate, transparent test, and the body-wall is often 

 beautifully necked with scarlet. If specimens are collected 

 immediately after a storm, they will be found to be still 

 alive, and the smallest of the bunch will show the beating 

 of the heart and the movements of the currents in a very 

 interesting way. Sea-squirts from deep water often have 

 very delicate tests, so that the internal structure shines 

 through clearly, but those found on the shore have usually 

 tough, resistant coats, which conceal the underlying organs. 



