92 LOLIGO PEALII. 



swimming and will therefore be termed the dorsal. The position of the 

 siphon will determine the apparent ventral surface. 



Technical Note. If you have an injected specimen supplied by some 

 marine laboratory the mantle cavity has probably already been opened. 

 If not, make a longitudinal incision a little to one side of the mid- 

 ventral line of the mantle, extending the cut from the collar nearly to 

 the posterior end of the body. Turn back the flaps of the mantle and 

 fasten them down in some way. Place in the dissecting pan, head 

 from you, and cover with water. 



THE MANTLE CAVITY. Study the organs exposed by the above dis- 

 section and observe that they are attached to the mantle only in the 

 mid-dorsal line and that the collar is entirely free from the head and 

 neck. Note the following: 



a) The pen, a hard structure whose thin edges show at the sides of the 



visceral sac occupying the posterior third of the mantle cavity. 

 It protects the other organs from injury on the dorsal side. Its 

 form and structure can best be studied in another dissection. 



b) The siphon widens out posteriorly and has on each side a bulging 



lateral pocket opening toward the mantle cavity. When dis- 

 tended with sea water these serve to close the opening between the 

 mantle collar and the neck. The approximation of the mantle 

 cartilages to the siphon cartilages also contribute to this end. 

 The former appear as a pair of longitudinal thickened ridges on 

 the mantle collar, the latter as a pair of trough-like depressions 

 on the sides of the siphon. 



c) Slit open the siphon at the outer end and look for a valve. Which 



way does the latter open ? By powerful rhythmic contractions of 

 the mantle water is alternately drawn in around the neck and 

 forced out at the siphon. Study out the principle by which 

 locomotion is thus caused. Which way will the animal move ? 

 Why do the rhythmic contractions go on when the squid is not 

 moving about ? How is the general shape of the body adapted 

 to the method of locomotion ? 



d) Note the heavy cord-like structures attached to the sides of the 



