THE ANIMAL KINGDOM 



19 



external simple epithelial ectoderm, 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. 



Of the two external apertures, the more ventral is the inhalent or 

 oral siphon and the other the exhalent or atrial siphon. The former leads 

 directly to the mouth, which is surrounded by a circle of tentacles. The 

 mouth leads into a greatly enlarged pharynx, which is perforated by 

 numerous gill-sHts or stigmata. The action of the cilia on the bars of 

 these slits serves to maintain a current of water from the pharynx into 

 the surrounding peribranchial or atrial cavity. Such relations resemble 



BRAIN 

 CILIATED FUNNEC 

 MOUTH 

 GILL SLITS 



/ATRIAL CAVITY/ SPINAL CORD 



\notochord 

 mntestine 



NENDOSTYLE "^"^ B. METAMORPHOSIS. 



Fig. 13. — Diagrams of stages in the metamorphosis of a urochordate larva. When 

 the larva settles down and becomes fixed by its adhesive papillae, the tail is lost and the 

 notochord disappears. Thus the chordate characters which are so evident in the larva 

 are partly lost in the mature organism. (Redrawn from Korscheldt and Heider, after 

 Seeliger.) 



those of similar organs in amphioxus. In the floor of the pharynx extends 

 a longitudinal groove, the endostyle, which morphologists generally 

 homologize with the thyroid gland of vertebrates. A somewhat similar 

 groove extends also along the dorsal side of the pharynx. The alimentary 

 canal consists of a short esophagus, a spherical stomach, and an intestine 

 which leads to an anus situated well forward in the atrial chamber. 



The heart lies ventral to the esophagus in the pericardial chamber. 

 There are no closed blood vessels, but the blood is pumped from the heart 

 forward to the pharynx in lacunar spaces the relations of which resemble 

 those of the afferent branchial vessels of vertebrates. The reproductive 

 organs lie in the loop of the intestine, posterior to the stomach. Their 

 ducts extend forward and open into the atrial cavity near the anus. 

 The gonads are hermaphroditic. 



