136 INVERTEBRATE ZOOLOGY 



portion of the body lying immediately between the two is the 

 dorsal side ; the opposite side, which is very much longer and 

 includes the surface of attachment, is the ventral side. A 

 stream of water is drawn into the incurrent opening, bearing 

 the minute organisms which constitute the animal's food and 

 the air needed for respiration; through the excurrent opening 

 the water is ejected, charged with fsecal matter and reproductive 

 products. 



Exercise l. Make a sketch of the animal on a scale of 2 or 3 ; 

 label the dorsal and ventral aspects and the siphons. 



Beneath the tunic and in contact with it is the mantle, which 

 is the remainder of the body-wall, the tunic being a highly modified 

 cuticula protecting its outer surface. Remove the entire tunic. 

 This may be easily done by snipping it with scissors and then 

 pulling it off with forceps; it is not tightly joined with the 

 mantle. The mantle will be seen to be a transparent structure 

 through which the internal organs appear. Observe the white 

 muscle bands in the mantle, especially the transverse and longitudinal 

 muscles in the siphons by means of which they are extended 

 and contracted. Note also the short tentacles at the incurrent 

 and excurrent openings. Count those at each opening. 



The digestive system. The most conspicuous internal organs 

 are the cream-colored genital glands near the center of the body 

 and the alimentary canal. The latter lies on the left side of 

 the body, where it appears as an S-shaped structure which 

 encloses the former. Place the body in water with the left side 

 uppermost and the siphons away from you, and study the 

 arrangement of the organs. The incurrent opening (at your 

 left) will be seen to have more prominent tentacles than the 

 excurrent opening. From the base of the incurrent siphon 

 the large pharynx, the most voluminous organ of the body and 

 the principal organ of respiration, will be seen extending to the 

 lower side of the body. Note the six longitudinal ridges which 



