404 UROCHORDA [CH. 



Sub-phylum III. TUNIGATA OR UROCHORDA. 



By many the group of Tunicata or Urochorda would be 

 considered the lowest portion of the phylum Vertebrata, and if we 

 had regard only to the adult structure this could not well be denied, 

 for in the adult hardly a trace of the Vertebrate relationship is 

 discernible. But the Tunicate commences life as a larva showing a 

 more developed structure in several important points than Amphioxus 

 possesses at any period of its life-history, and hence we must regard 

 the simple organisation of the adult as a degraded rather than a 

 primitive condition of affairs. 



The typical Tunicate larva is often called a tadpole because its 

 form recalls that of the well-known larva of the frog. 

 It attains a length of about a quarter of an inch, 

 and consists of a small round trunk and a thin vertical tail four 

 or five times as long as the trunk. The tail is the organ of loco- 

 motion, and is provided with a sheet of muscles on either side 

 by the alternate contraction of which powerful side-strokes are 

 executed and the animal is propelled forward (13, Fig. 200). The 

 tail is stiffened by a well- developed notochord which does not 

 extend into the trunk, hence the name "Urochorda" (Gr. ovpd, 

 tail ; xP% a string). A uniform flap of skin, a continuous 

 fin, forms a border to the tail. The trunk contains the enlarged 

 pharynx which opens by a narrow mouth in front : and laterally 

 communicates with the exterior by two ciliated openings the gill- 

 slits. Its ventral wall is swollen out into a pocket which causes the 

 under lip to protrude as a bulky chin. In this pocket we find 

 developed a ciliated groove, the en do style, having the same position 

 as the organ similarly named in Amphioxus. On the chin outside 

 are three peculiar warts which secrete a sticky slime and which are 

 used by the larva to fix itself to surrounding objects. The pharynx 

 leads behind into a short intestine which is attached to the ectoderm 

 high up on the side far in advance of the root of the tail. Hence 

 the process of shifting forward the anus and the corresponding 

 development of a purely muscular tail have been carried much 

 further in this tadpole than in Amphioxus. 



The nervous system in the tail is a simple neural tube ; but in 

 the trunk it expands into a thin walled vesicle, the so-called sense- 

 vesicle, which is the representative of the cerebral vesicle of Amphi- 

 oxus and the forerunner of the fore-brain of the higher Vertebrata. 

 As in the larva of Amphioxus, the sense- vesicle opens to the exterior, 



