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 feecal matter and reproductive 
products. 
Exercise 1. Make a sketch of the animal on a scale of 2 or 8; 
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 inthe 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 
