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 

 constant stream 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 

 " 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. 



FIG. 96. Ascidian or sea-squirt. 



