CHAPTER X. 



THE TUNICATA OR ASCIDIOIDA. 



This remarkable and, in many respects, isolated group of 

 marine animals contains both simple and composite, fixed and 

 free, organisms. None attain a length of more than a few 

 inches, and some are minute and almost microscopic. 



The simplest members of the group, and those the struct- 

 ure of which is most readily comprehensible, are the Appen- 

 dicularice y minute pelagic organisms, which are found in all 

 latitudes, and are propelled, like tadpoles, by the flapping of 

 a long caudal appendage at the surface of the sea. 



Appendicularia flabellurn (Fig. 147) has an ovoid or flask- 

 shaped body (A), one-sixth to one-fourth of an inch in length. 

 The appendage (JS) is from three to four times as long as the 

 body, to one face of which it is attached near, but not at, the 

 posterior extremity. It is flattened, and is supported by a 

 firm central axis, which may be termed the urochord (Fig. 

 147, /). The greater part of the body is usually invested by 

 a structureless gelatinous substance, but, on its rounded 

 hinder extremity, this ceases to be distinguishable from the 

 ectoderm. 



On the caudal appendage the polygonal contours of the 

 cells of which the ectoderm is composed are plainly discern- 

 ible. 



The mouth has an overhanging lip. It leads into a large 

 pharyngeal sac, the walls of which are formed by the endo- 

 derm. Posteriorly this sac narrows into the oesophagus, 

 which bends toward the haamal side of the body, and then 

 opens into a spacious stomach, which takes a transverse direc- 

 tion, and is divided into two lobes, a right and a left. 



From the left lobe the intestine arises, and, bending in- 

 ward, turns abruptly forward in the middle line, where it 

 terminates midway between the oral aperture and the attach- 

 ment of the caudal appendage. The intestine, therefore, has 



