2O Invertebrate Zoology. 



If the living animal is irritated, the tentacles are with- 

 drawn, the disc is lowered, and by the contraction of the 

 ring-muscle, the walls of the column are drawn together 

 over the free end. If the contraction goes still further, 

 numerous white, thread-like bodies, acontia, are seen to 

 ooze from the cinclides. 



Make a drawing of the contracted animal. 



Internal Anatomy. With a sharp knife divide the 

 animal from disc to base, through a plane at right angles 

 to the stomodaeal opening, and then remove from one of the 

 halves the thin tissue of the base. The mouth will be 

 found to lead into an elongated, thick-walled, and more or 

 less wrinkled tube, the oesophagus. Is its wall in any way 

 differentiated at the siphonoglyphes? Below, the oesopha- 

 gus opens directly into the coelenteric chamber, a general 

 cavity limited externally by the walls of the column and 

 divided, or partially divided, into numerous smaller cham- 

 bers by delicate, radially arranged partitions, the mesenteries 

 or septa. Only six pair of mesenteries extend entirely across 

 the coelenteric chamber and unite with the oesophagus. 

 They are called the primary mesenteries, and are definitely 

 arranged. Two pair of primary mesenteries, the directive 

 septa, occupy the plane of the longer axis of the mouth, 

 and are attached to the walls of the siphonoglyphe. They 

 divide the animal into two lateral halves. Between each 

 pair of primary mesenteries indeed, between any pair of 

 mesenteries is a limited chamber, the infra-radial cham- 

 ber, while between two pair of mesenteries are the inter- 

 radial chambers. There are six primary inter-radial 

 chambers, three on either side. Between the primary 

 septa are the secondary and tertiary septa, partially divid- 



