PHYLUM CHORDATA 



323 



After a short existence as a free-swimming 

 organism, the larva becomes attached to 

 some object by three projections on the an- 

 terior end, which secrete a sticky fluid. It 

 then undergoes a retrogressive metamor- 

 phosis, during which the tail with the noto- 

 chord disappears, and the nervous system is 

 reduced to a ganglion. 



The typical adult tunicate (Fig. 205) is 

 attached by a base or stalk and surrounded 

 by a thick, tough, elastic membrane, the 

 tunic. This is composed of a celluloselike 

 substance, a material rarely found in ani- 

 mals but common in plants. The tunic is 

 lined by a membranous mantle which con- 

 tains muscle fibers and blood vessels. At the 



distal end are two external openings: one 

 is the "mouth" or incurrent siphon (bran- 

 chial opening), into which a current of 

 water passes; the other is the excurrent 

 siphon (atrial opening) through which the 

 water escapes to the outside. This current 

 of water brings food into the digestive tract, 

 furnishes oxygen for respiration, and carries 

 away gametes and excretory substances. 

 Within the test and mantle is the atrial cav- 

 ity which contains the internal organs. At the 

 base of the incurrent siphon is a velum, 

 forming a sensory sieve, through which in- 

 coming water and food must pass. Below the 

 velum is a pharyngeal sac; on the ventral 

 side of this is the endostyle, a pharyngeal 



Mouth 



Dorsal nerve cord 

 Notochord 



Ojl 



Adhesive 

 papillae 



Intestine 

 Gill slit 



Figure 206. The free-swimming larva of a tunicate, showing all three chordate characteristics: 

 notochord, dorsal nerve cord, and gill slits. 



groove, lined with mucous and ciliated cells. 

 Microscopic plants and animals are en- 

 tangled in mucus secreted by the endostyle 

 and move downward into the esophagus. 

 This leads to the stomach which connects 

 with the intestine. The digestive tube is 

 bent upon itself and opens into the atrial 

 cavity through the anus. A digestive gland 

 connects by a duct to the stomach. A single 

 nerve ganglion lies between the two siphons, 

 with nerves to various structures. Located 

 near this is a neural gland which may have 

 an endocrine function. 



The circulatory system consists of a tubu- 

 lar heart, to each end of which is connected 



a large vessel, and each of the vessels gives 

 off branches to various structures. One un- 

 usual feature of the circulation in these 

 forms is that the direction of blood flow is 

 reversed at short intervals. 



Tunicates are hermaphroditic, but they 

 are usually self-sterile so that sexual repro- 

 duction requires two animals. Some species 

 also reproduce asexually by budding. 



Subphylum Vertebrata 



The Vertebrata are chordates having a 

 segmental backbone or vertebral column. 

 They also possess an axial notochord at some 



