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- 



.bd- 



nata 



FIG. 31. An individual 

 belonging to the class 

 Larvacea (Oikopleurd) 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; ht, heart; int, intestine; ne, nerve; 

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

 gn '", first two ganglia of nerve of tail; noto, notochord; oes, oesophagus; or. 

 ap, oral aperture; oto, otocyst (statocyst); peri, bd, peripharyngeal band; 

 ph, pharynx; tes, testis; stig. 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 evidences 

 in the U-shaped intestine that the Larvacea have come 

 sessile ancestor. It is because we consider the Ascidiacea as 



a 





