iv PROBOSCIS OF THE NEMERTINA 197 



muscles are always more strongly developed, attach themselves at both 

 ends to the integument, and serve chiefly for drawing in the anterior 

 end with the wheel organ, for shortening the tail or foot, and, in tubi- 

 colous forms, for withdrawing the body into its case. 



The dermo-muscular tube of the Chcetognatha, which swim with 

 arrow-like speed, is very strongly developed, in keeping with its high 

 degree of activity. A circular muscle layer is, however, wanting. The 

 longitudinal musculature (Fig. 130, Im) is divided by 4 longitudinal 

 lines of interruption (2 lateral, 1 medio- 

 dorsal, and 1 medio-ventral) into 4 areas 

 (2 dorsal and 2 ventral). 



"Worm-like contractions do not occur, in con- 

 sequence of the want of a circular muscle layer. 

 By alternate contractions of dorsal and ventral 

 musculature, and by the co-operation of the hori- 

 zontal fins, the elastic body is quickly propelled 

 forwards. It was chiefly the similarity of the 

 muscular arrangement of the Chcetognatha with 

 that of the Nematoda which caused many zool- 

 ogists to place the former with the Nemathelminths. 



TV, +>io nrorrnc ^nnnrlellrt tlim*A i a fTn'n FlG ' 130. Transverse section 



In the genus bpadella t re is a tnm through the trunk of a Sagitta (after 

 layer of transverse muscle fibrillae lying o. Hertwig). ih, Body cavity ; mes, 



in the body cavity, closely applied to the mesentery of the intestine ; md, mid- 



ventral musculature. _ The arrangement ^,3^' " rasoulat " re; 

 of the musculature in the head of the 



Chcetognatha undergoes a marked complication. The most important 

 head muscles are those which serve for moving the seizing hooks. 



IV. The Proboscis of the Nemertina and Aeanthoeephala. 



These organs may be treated of together, although they arise quite 

 independently, and are not homologous. 



In the proboseidal apparatus of the Nemertina (Fig. 131) we dis- 

 tinguish the following principal parts: (1) the proboscis sheath; (2) 

 the proboscis ; (3) the retractor muscle of the proboscis. 



The proboscis sheath (rs) is a tube closed on all sides which lies 

 above the intestinal canal in the parenchyma of the body. Its 

 muscular walls consist principally of a circular and a longitudinal 

 muscle layer. The proboscis sheath stretches more or less far back, 

 often to near the hindermost end of the body. 



The proboscis (r) is also a cylindrical tube. It lies invaginated 

 into the proboscis sheath. The space, closed in on all sides, between 

 proboscis sheath and proboscis is filled with fluid. The walls of the 

 proboscis sheath and of the proboscis join each other not far behind the 

 foremost end of the body. From this point a short tube (rhyneho- 



