50 



VERTEBRATE ZOOLOGY 



pennanently rooted to one spot. Some of the free-swimming forms 

 (Larvacea) are interpreted as psedogenetic forms, wliicli liave retained 

 the larval condition throughout life; others are evidently derived 

 secondarily from sessile ancestors and now live a free pelagic life. 

 The composite illustration (Fig. 21) shows the general appearance of 

 many common types of urochordates. The sessile type seems to Ije 

 representative of, and will serve to illustrate, the essential features of 

 the whole sub-phylum. 



Order I. Ascidiacea 



A TYPICAL ASCIDIAN 



An external view of an ascidian (Fig. 22) reveals little of interest. 

 It does not even look like a living creature; much le.ss a chordate. It 

 is a dull brown oljject resembling a leather bag 

 or bottle witli two short necks, one terminal and 

 one somewhat on the side, the former Ijeing the 

 oral funnel and the; latter the atrial funnel or atrio- 

 'pore. If one watches the water currents care- 

 fully ho will observe that the water enters the 

 oral funnel in a stead j^ stream and exits from 

 the atriopore. The wrinkled brown covering or 

 tunic (which gives the name Tunicata to the 

 group) is merely a lifeless protective layer com- 

 posed of animal cellulose, a substance ahnost 

 identical with wood. The body wall within the 

 tunic is composed largely of glands and con- 

 nective tissue and shows neither myotomes nor 

 any other segmented structures. The tunic and 

 bofly wall of olie side r(>moT,-ed, we get a view of 

 the interior (Fig. 23). What we find is little 



Fifi. 22.— External moi-g than an exaggerated food-concentralinq ap- 



appearance of a tyiji- ^ , , ' , , . , , „ , . 



cal Ascidian (.4 .scirfia) paratus, closely comparable with that of Amphi- 



seen from the risht oxus. The circular oral funnel opens through a 



side. (Fro,,, Farker ,eUan, with velar tentacles, into a great sac^-like 



and llaswell, alter , ., , . . "^ 



Herdman.) jjlutrijn.v that occupies far more than its share 



of tli(> avtiihiblc space. This pharynx is an 



ehihorate sieve wilh coimlle.ss small slit-like openings, stigmata, which 



are subdivided phaiyiigeal clefts. On the ventral side there is an 



