408 UROCHORDA OR TUNIC ATA. 



obliterated. Two ectodermal invaginations form the originally double 

 peribranchial chamber, and the primary gill slits break through the wall 

 of the pharynx. 



For some hours the tadpole-like larva enjoys a free swimming life, 

 using its tail as an organ of locomotion. Then it fixes itself by a papilla 

 on the head, and begins almost immediately to degenerate. The tail 

 shrinks and disappears, the notochord being consumed by phagocytes. 

 The nerve cord is lost, and with it the larval sense organs. The 

 pharynx and peribranchial chamber assume their adult form, and the 

 whole animal undergoes a metamorphosis, which one of the most 

 signal instances of degeneration. 



FIG. 131. Section of newly fixed larva of Clavellina. 

 (After SEELIGER.) 



z., Inhalent aperture ; ft. , ciliated groove; sb., sensory vesicle ; e., 

 exhalent aperture; r., posterior part of medullary canal; ck., noto- 

 chord; ^., heart; ks., gill slits;/., peribranchial space; es., endo- 

 style ; hp., attaching papillae. 



Classification. 

 Order i. LARVACEA or PERENNICHORDATA. 



Appendicularia, Oikopleura, Fritillaria, Kowalevskia. 



This order includes a few simple pelagic forms, which exhibit many of 

 the characters of the larvae of the Ascidians. All are minute, and 

 furnished with a large locomotor tail which is bent forwards at an angle 

 to the body. Epidermic cells near the mouth secrete a slimy but 

 consistent test, or " house," which is frequently abandoned and formed 

 anew. The tail has a supporting notochord, and very large muscle 

 cells. The nervous system consists of a lobed ganglionic mass above 

 the mouth, from which a nerve cord is continued backwards and along 

 the tail ; this is furnished with other ganglia in the tail region. It lies 

 to the side of the notochord, and like the ganglia is said to be furnished 

 with an axial canal. In connection with the cerebral ganglion there is 

 a pigment spot, an otocyst (auditory ? ), and a tubular process com- 

 municating with the pharynx, and corresponding to the subneural gland 

 and the ciliated duct of other Tunicates. There are two gill slits which 

 communicate with two ectodermic atrial invaginations, as in the early 

 larval stages of Ascidia. The mouth is almost at the anterior end, 

 the anus at the root of the tail. The heart is very simple, and there are 

 no distinct vessels. The hermaphrodite reproductive organs lie 

 posteriorly and are ductless. The eggs are difficult to obtain, and in 

 consequence little is known of the development. 



