GENERAL ZOOLOGY 



Mesenchyme 

 cells 



Muscle fibers 

 Nerve ring 



Marginal 

 cilia 



Fig. 11.23. Pilidium larva of a nemertine; optical section, showine; distribution of nerve and 

 muscle fibers. (Redrawn from W. Salensky, 1886, ^eilschrifl fitr wissenschaftliche ^ooloate, 

 vol. 43.) 



The structure of a representative nemertine is shown in Figure 11.22. The 

 ciliated body is somewhat flattened dorsoventrally, with a mouth at the 

 anterior end and an anus at the posterior end. A protrusible structure called 

 the proboscis, lying in a sheath dorsal to the digestive system, is used as a 

 tactile organ and in some species for the capture of prey. Unlike the 

 proboscis of planarians, which is actually the pharynx, the proboscis of the 

 nemertines is not an integral part of the digestive system, although in some 

 types its cavity opens into the mouth. Food captured by the proboscis is 

 brought to the mouth and ingested, and the proboscis is then retracted into 

 its own dorsal sheath. 



The alimentary canal in nemertines is a straight tube, leading from mouth 

 to anus, usually with paired lateral pouches. The space between the internal 

 organs is occupied by mesenchymal tissue similar to that of turbellarians; 

 hence there is no coelomic cavity. The excretory system consists of pro- 

 tonephridia resembling those of Platyhelminthes but more specialized. A 

 notable advance characteristic of nemertines is the presence of a blood- 

 vascular system. This consists of a median dorsal vessel, and on each side 

 a lateral longitudinal vessel. These unite at each end of the body. The 

 blood plasma and blood cells are usually colorless. The nervous system con- 

 sists of two pairs of ganglia, constituting the "brain," lying anteriorly and 

 dorsally, and three longitudinal cords. Two cords are lateral and one dorsal, 

 and they are connected at various levels by commissures. A number of 



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