THE PHYLUM CHORDATA 



59 



merely a temporary structure that serves for a night's lodging, as it 

 were; for the animal leaves it at intervals and is capable of making 

 another "house" in an hour or so. The tail 

 is rather loosely jointed to the body and 

 serves as a flexible paddle in locomotion. 

 The body is like that of a tunicate, with the 

 whole food-concentrating apparatus reduced 

 and simplified (Fig. 32). There is only one 

 pair of pharyngeal clefts that open directly 

 through paired atriopores; a condition sug- 

 gestive of the larval atrial cavities in the 

 ascidians. The endostyle also occupies an 

 anterior position similar to that of a larval 

 Amphioxus. 



In almost every respect the Larvacea sug- 



Fig. 31. — An individual 

 belonging to the class 

 Larvacea (Oiko'pleura) in 

 its gelatinous "house." 

 Only the small hammer- 

 shaped object in the main 

 passage-way is the animal 

 itself. The arrows show 

 the current of water 

 through the "house." 

 (From Herdman after 

 Fol.) 



Fig. .32. — Diagram of a larvacean (Appendicularia) from the right side, with 

 most of the tail removed, an, anus; hi, heart; int, intestine; 7i,e, nerve; 

 ne', candal portion of nerve; ne. gn, principal nerve ganglion ; ne. gn ", rie. 

 gri '", first two ganglia of nerve of tail; nolo, notochord; oes, oesophagus; or. 

 ap, oral aperture; olo, otocyst (statooyst); peri, bd, peripharyngeal band; 

 ph, pharynx; tes, testis; sUg. one of the single pair of pharyngeal clefts, stigmata; 

 stom, stomach. (From Parker and Haswell, after Herdman.) 



gest a permanent larval condition akin to neotony or psedogenesis. 

 The alternative view, that these forms are prototypic of the ances- 

 tral chordate, is, we believe, not well taken; for there are evidence? 

 in the U-shaped intestine that the Larvacea have come from a 

 sessile ancestor. It is because we consider the Ascidiacea as most 



