A SQUID 105 



Note that the arms may be divided into a right and a left group, 

 each containing five arms. Observe a single arm ; how many- 

 rows of suckers has it ? Observe the structure of a sucker. Note 

 the difference between the two long arms and the others in the 

 place of origin and the arrangement of the suckers. 



The Mantle Cavity. Open the mantle cavity by a longitudinal 

 incision through the thick mantle wall of the under side of the 

 body to one side of the median line, running from the collar to 

 the apex of the animal, taking care not to injure the delicate 

 organs within. Notice, in the first place, that the collar is not 

 attached to the head at any point of its circumference ; and also 

 that on the inner surface of the mantle, on the upper side of the 

 body in the median Hne and also on each lateral surface, there is 

 an elongate, cartilaginous structure which fits into a correspond- 

 ing cartilage on the body — an arrangement which enables the 

 collar to be applied very closely to the head. 



Place the animal in water with the head away from you and 

 pin down the flaps of the mantle. Observe the soft visceral mass 

 within it, and notice that it is fused with the mantle only in the 

 median Hne of the back ; also that the pen, which is embedded 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 communi- 

 cating between the mantle cavity and the outside. Slit it open and 

 observe the flapHke valve at the forward 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 out- 

 side 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. 



