50 



VERTEBRATE ZOOLOGY 



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

 (Larvacea) are interpreted as paedogenetic forms, which have 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 be 

 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 less a chordate. It 

 is a dull brown object resembling a leather bag 

 or bottle with two short necks, one terminal and 

 one somewhat on the side, the former being the 

 oral funnel and the latter the atrial funnel or atrio- 

 pore. If one watches the water currents care- 

 fully he will observe that the water enters the 

 oral funnel in a steady 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 almost 

 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 

 body wall of one side removed, we get a view of 

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



FIG. 22. External more than an exaggerated food-concentrating ap- 



Sfftaft&fcUrftt!) P aratm ' closel y comparable with that of Amphi- 

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



side. (From Parker velum, with velar tentacles, into a great sac-like 

 and Haswell, after 7 ,-, , f ,-, , i 



Herdman ) pharynx that occupies far more than its share 



of the available space. This pharynx is an 

 elaborate sieve with countless small slit-like openings, stigmata, which 

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



