THE CHORD ATA 137 



the Atrial chamber. Into this the gills pass the 

 sea-water which they have breathed. 



The group, as a whole, is sometimes consid- 

 ered to present evidence of having degenerated 

 from a higher type ; but whatever else may be 

 doubtful or obscure in its history, the nature of 

 the larval notochord is quite clear and certain ; 

 zoologists have never had any doubt about its 

 nature since the first few years after its discovery. 



Ascidians are not at all uncommon animals on 

 the English coast. Some of them may be met with 

 on stones near low-water mark, and I have often 

 seen them on the shells of oysters sold in the 

 shops for there the town - dwelling naturalist 

 may often find a good many interesting things 

 without much trouble. They are like little lumps 

 of tough jelly ; of various colours, according to 

 the kind, red being the most common, and of very- 

 indefinite shape. You may see some of the co- 

 lonial kinds forming pretty star-shaped patterns, 

 attached to various objects, such as stones and 

 the larger seaweeds. 



The place of Ascidians in classification was a 

 puzzle, until their relationship with Vertebrates 

 was discovered. At one time they were placed! 

 with the Mollusca. Now they are grouped, to- 

 gether with the Vertebrata and some other crea- 

 tures that remain to be spoken of, under the 

 name of Chordata, or animals possessing a Noto- 

 chord. 



Some of the Ascidians present what has been, 

 already described in other types (p. 57) as "al- 

 ternation of generations." The discovery of this- 

 fact was made by the poet Adelbert von Chamisso. 

 Some of his verses are known to English readers,, 

 for whom they were translated by Mary Howitt, 



