CLASSIFICATION 481 



tubes which unite the branchial sac of one individual to that of 

 its neighbours. 



Among the five hundred or more Simple Ascidians the fol- 

 lowing British forms may be mentioned : Clavelina lepadifor- 

 mis and Perophora listeri, common Clavelinids in shallow 

 water off the coast ; Ciona intestinalis and Ascidia mentula, 

 two of the commonest of our Ascidians ; Corella parallelogramma, 

 common in the zone of coralline Algae, with a crystalline test 

 through which the beating of the heart and the like can be 

 readily seen; Styelopsis grossularia, found under stones at low 

 tide, and sometimes known as " the red-currant squirter " ; 

 Molgula oailata with a test of a walnut's size covered with 

 gravel or shell and Engyra glutinans of a small hazel-nut's 

 size, almost covered with sand, both living freely on the floor 

 of the sea at a depth of ten to thirty fathoms. I have col- 

 lected many specimens of the last-named in dredging off the 

 Arran coast. 



Very interesting in a simple way is the adaptation often 

 seen in Simple Ascidians which live in the deep sea. They are 

 raised on relatively long stalks which bear them up beyond the 

 risk of being smothered in the fine ooze. This adaptation may 

 be seen in Hypobythius calycodes from 2900 fathoms ; in Cory- 

 nascidea, Herdman, in Culeolus, Herdman, which is represented 

 at 2425 fathoms ; in Boltenia, Styela, and other forms. At 

 various points in the animal kingdom we find the same simple 

 adaptation to abyssal life, the development of a stalk to lift the 

 animals out of the mud — in sponges, like the glass-rope sponge 

 {Hyalonemd); in Alcyonarians, such as Umbellula ; in Anti- 

 patharians which belong to the Sea-Anemone alliance ; and in 

 Crinoids or Sea-Lilies. It is this aspect of things which this 

 book is intended to illustrate, and although Tunicates are not 

 very familiar animals, nor very attractive to those who do not 

 know them, nor exhibiting much in the way of habits, they 

 may serve to show how the members of a rather small class 

 are adapted to very varied modes of life — in the open sea, on the 

 shore, and in the great abysses. 



The Compound Ascidians form by budding compact 

 masses in which the individual members or " ascidiozooids " are 

 buried in a common investing substance and have no separate 

 tests. As we have seen, they are linked to the Simple Ascidians 



31 



