150 HALF AN HOUR WITH SEA-MATS AND SQUIRTS. 



If we open them, we find a nervous membrane lining 

 this leathery case, and fixed to it at two points, corre- 

 sponding to the two separate openings, the one to take 

 in, the other to eject the water." In this respect, the 

 anal and mouth tubes correspond to the siphons we 

 see in such bivalve shells as Venus. In Fig. 79 we have 

 an illustration of the commonest and most abundant 

 of our British species, which, however, can only be 

 met with in tolerably deep water, and therefore 

 must be looked for in the rubbish brought in by 

 the trawling-boats. It ranges in depth from low 

 water to about twenty fathoms, being most abundant 

 off the mouths of large rivers, where it is so common 

 as to fill the nets of the fishermen, and cause them 

 no small annoyance. Although attached by its base 

 to some object along the sea-bottom, it is easily 

 loosened, especially by such a powerful implement 

 as the trawl. Mr. Wood describes them as " looking 

 to all the world like white hot-house grapes," when 

 first brought up. With many of the fishermen the 

 belief is strong that they are nothing more or less 

 than congealed water ! One very striking peculiarity 

 belongs to the Ascidians the outer tunic or coat 

 contains a substance called cellulose, which was before 

 deemed peculiar to the vegetable kingdom, and 

 indeed has only been found out of it in this animal. 

 Dr. Woodward thinks it may be due to the fact the 

 Ascidians feed on diatoms and minute vegetation. 

 The same coat also contains rude spicules or concre- 

 tions, like those found in Gorgonias and " dead 



