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. 



