Classification of Cliordates: Protochordata 



397 



STATIC ORGANy 



PHARYNX 

 GILL SLITS 



Fi<;. 306. Diagram of a larval urochordate. The similarity of the larval uro- 

 chordate to the embryo of a cephalochordate (Amphioxus) suggests that the uro- 

 chordate is near the main line of vertebrate ancestry. (After von Beneden and 

 Julin. Courtesy, Neal and Rand: "Chordate Anatomy," Philadelphia, The 

 Blakiston Company.) 



body such as occurs in vertebrates, and the digestive tube becomes 

 bent upon itself so that eventually the anus lies near the mouth. The 

 mat ure larva swims actively by means of its long, muscular tail and, in 

 a general way and except for its comparatively minute size, strongly 

 resembles the tadpole of a frog or toad. 



In the great majority of tunicates, the larva eventually attaches 

 itself to some submerged object by means of the anterior adhesive 

 papilla. The tail, together with the notochord and the greater part of 

 the dorsal nerve-cord, is absorbed (Fig. 307). These and various other 

 changes combine to transform the larva into the saclike adult (Fig. 

 308). 



The general anatomy of the adult of such typical sessile tunicates 

 as Ascidia, Ciona, and Molgula is represented in Fig. 309. The outer- 

 most layer of the animal is a "test" or "tunic" whose substance is 

 secreted primarily by the skin. The essential constituent of the tunic is 

 a substance, tunicin, which is chemically similar to the cellulose of 

 plants. Beneath the test, and connected with it loosely, except in the 

 region of the two apertures of the body, lies the body-wall or " mantle." 

 This consists of an external simple-epithelial layer, and, beneath this, 

 connective tissue containing a network of muscle-fibers which are more 

 abundant in the region of the two apertures of the body, which they 

 serve to close and open. 



In the ventral region of the median plane of the animal, the deeper 

 tissue of the body-wall extends inward and is joined to the wall of the 

 pharynx, but on each (right and left) side of the pharynx an extensive 

 space intervenes between it and the body-wall. Dorsally, these two 

 spaces join into a large median space which leads to the exterior 

 through the dorsal "siphon." These spaces collectively are known as 

 the peribranchial or atrial cavity. Its median division, into which the 

 anus and the genital ducts open, is sometimes called a "cloaca." The 

 atrial cavity is not to be confused with a coelom, which it deceptively 

 resembles. 



