lafe 3'ears, and Ascidians are now regarded as the degener- 

 ate descendants of a very lowly-developed group of the 

 early Vertebrata (or more correctly, Chordata). 



But before we are in a position to understand this 

 important matter, it is well to have some knowledge of 

 the ascidian structure at several different stages in the 

 life-history, and we shall connnence with the last stage 

 of all — the full-grown, or adult condition. 



Mode of Occurrence. 



Ascidians are all marine, and they have been found in 

 all seas, from tropical to polar regions, and at depths 

 varying from near higli-water mark down to three and a 

 quarter miles or so. Round most of our own coasts they 

 are common, and some kinds are familiar enough, under 

 the name of " sea-squirts," to many sea-side visitors, as 

 being amongst the inhabitants of rock-pools which can be 

 collected and kept in aquaria. Such forms are usually 

 found as dome-shaped l)odies of a dull red colour, adhering 

 to the rock or sea-weed, and having two small openings on 

 their upper surface from which, when touched, they emit 

 delicate jets of sea-water with considerable force — thus 

 establishing their claim to the title of " sea-squirts " ; and 

 their resemblance to double-necked leather bottles, whence 

 the name Ascidian (from the Greek ao-Kos). 



Others ngain form flat gelatinous expansions attached 

 to sea-weeds or stones, and symmetrically marked with 

 bright spots of colour in the form of circles, meandering 

 lines, or starlike patterns. These are really colonies in 

 which each spot of colour or ray of the star represents an 

 ascidiozocid or member of the colony equivalent to the 

 whole animal m the case of the solitary Ascidian. 



By dredging around the coast, outside low-water mark, 

 many other kinds of Ascidians are found, of diverse forms 



