162 



INVERTEBRATE MORPHOLOGY. 



IV. Class Nemertina. 



The three preceding classes show marked evidences of 

 genetic affinity, the characteristic differences of structure in 

 the Trematodes and Cestodes being due to the parasitic 

 habits of these forms. The Nemerteans, on the other hand, 

 though apparently tracing descent from a Turbellarian-like 

 ancestor, show a marked advance in structure, and must be 

 regarded as organisms of a considerably higher grade than 

 the other Platyhelminths. 



They are for the most jDart marine, though a few forms 

 inhabit fresh water or even damp earth, 

 and are usually elongated ribbon-like 

 forms, reaching a length, in some cases, 

 of several centimetres. The body is ex- 

 ternally unsegmented, though a more or 

 less marked metamerism of the internal 

 organs, due to their repetition at definite 

 intervals, may be present. The ectoderm 

 of the body resembles that of the Tur- 

 bellaria in being throughout ciliated, and 

 rests upon a basement membrane, which 

 in some cases contains cells. Within 

 the membrane are a varying number of 

 muscle layers, differentiations of the 

 outermost portions of the mesodermal 

 tissue, which in the form of a parenchyma 

 occupies the interval between them and 

 Fig. 84— Anterior PoR- tlie digestive tract. This (Fig. 84, d) is 

 TioN OF Nbmektban. an almost straight tube, except in the 

 genus Malacohdella, and is pushed out on 

 each side into sac-like pouches, which 

 are arranged in some cases with a 

 regularity of succession almost meta- 

 meric. It opens to the exterior at the 

 anterior end of the body by the mouth, 

 a short non-muscular oesophagus inter- 

 vening between the intestine proper and that opening ; and 

 at the other end of the body is a second communication with 



ce = cerebral ganglion. 

 eg = ciliated funnel. 

 d = intestine. 

 oc = eyes. 

 011 = ovary. 

 pr = proboscis. 

 rm = retractor muscle of 

 proboscis. 



