152 ANIMAL FORMS 
which (the lancelets, lampreys, fishes, amphibians, reptiles, 
birds, and mammals) are true vertebrates, while the others 
embrace several peculiar animals of much simpler organiza- 
tion. 
144. The ascidians.—Among the latter are a number of 
remarkable species belonging to the class of ascidians or 
sea-squirts (Fig. 96). 
These are abundantly 
represented along our 
coasts, and are readily 
distinguished by their 
sac-like bodies, which 
are often attached at 
one end to shells or 
rocks. On the opposite 
extremity two openings 
exist, through which a 
constantstream of water 
passes, bearing minute 
organisms serving as 
food. When disturbed 
they frequently expel 
the water from: these 
pores with considerable 
force, whence the name 
Fig, 96.—Ascidian or sea-squirt. ee sea-squirt.” While 
many lead solitary lives, 
numerous individuals of other species are often closely 
packed together in a jelly-like pad attached to the rocks, 
and others not distantly related are fitted to float on the 
surface of the sea. 
The young when hatched resemble small tadpoles both in 
their shape and in the arrangement of some of the more 
important systems of organs. For a few hours each swims 
about, then selecting a suitable spot settles down and ad- 
heres for life. From this point on degeneration ensues. 
