. The chordates. Chordata. 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 sw
. The chordates. Chordata. 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 " ; 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 t
Size: 2794px × 894px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollect, bookpublisherphiladelphiablakiston
