126 INVERTEBRATE ZOOLOGY 
mass Within it, and notice that it is fused with the mantle only 
in the median line of the back; also that the pen, which is 
imbedded in the mantle, protects the viscera on that side. 
Observe the siphon and probe it. It will be seen to be a funnel- 
shaped tube communicating between the mantle cavity and the 
outside. Slit it open and observe the flap-like valve at the for- 
ward end. Notice the lateral pockets on each side of the siphon 
which open toward the mantle cavity and occupy the space 
between the siphon and the median line of the back. They 
are separated from the siphon by the lateral cartilaginous rods 
above mentioned. It will be seen that while water can easily 
pass into the mantle cavity from the outside all around the 
neck, a contraction of the muscular wall of the mantle would 
force the water out through the siphon only, as that which is 
forced into the lateral pockets would at once swell them out 
and close the spaces at the sides of the siphon. It is, in fact, by 
thus shooting the water in the mantle cavity forcibly through 
the siphon that the animal swims. 
Note the two large retractor muscles of the siphon and beneath 
them the two larger retractor muscles of the head. 
Observe again the visceral mass; it is covered by a thin, 
transparent membrane, the body-wall, the extreme thinness of 
which is correlated with the thickness of the mantle which 
covers it. If the animal be a female that fact may be known 
by the presence of two very large, transversely striated bodies, 
called the nidamental glands, which lie near the center of the 
body, and are a part of the reproductive system. Carefully 
remove these in order to expose the organs beneath. If the 
animal be a male (and the student should obtain a male if pos- 
sible), it can be recognized by the absence of nidamental glands 
and also by the presence of the testis, a large white tubular 
organ which lies near the median line toward the hinder end of 
the animal. In the female the ovary, which occupies a similar 
position, is often very full of the granular ova. 
