8 MORPHOLOGICAL TYPES 



Seen from the side and visible through the skin they are 

 markedly V-shaped, with the apex pointing forwards. Those 

 of one side alternate with those of the other. 



The ventral side of the front end of the animal is expanded 

 to form the oral hood. The sides of the oral hood (which 

 are not quite symmetrical) bear a fringe of buccal cirrhi, each 

 one supported by a small jointed skeleton. The cirrhi bear 

 sense-organs. On the under surface of the oral hood the 

 epithelium is modified in places into a ciliated organ (the 

 organ of Miiller, or wheel-organ) whose function it is to 

 create a current of water flowing towards the mouth. Slightly 

 to the right of the middle line, a small depression opens into 

 the cavity of the oral hood, known as Hatschek's pit. (The 

 development of this interesting structure is described on 

 p. 167.) 



The mouth is situated at the hind end of the oral hood, 

 and is a circular aperture pierced through a vertical transverse 

 plate, the velum. The size of the opening is regulated by a 

 circular sphincter muscle. In addition, there are twelve 

 velar tentacles arising from the rim of the mouth, provided 

 with sense-organs. Their function is to act as a strainer 

 across the mouth-opening. 



The anus opens not at but near the hind end of the body 

 on the ventral surface, slightly to the left of the middle line 

 owing to median position of the ventral fin. Just in front of 

 the ventral fin is another aperture, the atriopore, the signifi- 

 cance of which will be understood with a knowledge of the 

 structure of the atrium. 



Other external features to be noticed are the olfactory (or 

 Kolliker's) pit on the left side of the body very near the front 

 end, and the metapleural folds of the atrium. The gonads 

 can also be seen from the outside, as a row of sacs between 

 the mouth and the atriopore. 



Alimentary System.— The gut leads straight from the 

 mouth to the anus without any loops or kinks. The anterior 

 half of it is the pharynx, the posterior is the intestine. A 

 blind outpushing is given off on the right side from the front 

 of the intestine, forming the so-called liver- diverticulum. 



