VI] CLASSIFICATION 117 



Many species of Nemertine are found on the British coast. 

 As examples we may mention Linens (Fig. 48), a long thin form 

 without a stylet in the proboscis, and Tetrastemma, a short broad 

 form with four eyes and a stylet in the proboscis. A remarkable 

 land Nemertine Geonemertes is found in Australia. It moves by 

 ejecting the proboscis coiling it round a twig and then pulling the 

 rest of the body after it. 



The Nemertines, so far as our present knowledge extends, form 

 a completely isolated group in the "animal kingdom. 



From the circumstances that some Rhabdocoel Turbellarians 

 have a protmsible organ in the anterior part of the body, and that 

 the excretory organs of Nemertines bear a certain resemblance -to 

 those of Platyhelminthes, it has been supposed that Nemertines and 

 Platyhelminthes are allied. The presence of an anus in Nemertines 

 tells strongly against this view, since the change from a single 

 opening to the alimentary canal to two openings, one for ingestion 

 of food and one for rejection of faeces, was a momentous step in 

 evolution and must have required a long period for its completion. 

 The absence of all accessory genital organs in Nemertinea as 

 contrasted with their presence and elaboration in Platyhelminthes 

 tells very strongly against the mutual affinity of the two groups. 



The Nemertinea are divided into classes in accordance with the 

 condition of the nervous system and the arrangement of the layers 

 of muscles in the skin. These classes are as follow : 



Class I. PROTONEMERTINI. 



A nervous sheath underlies the entire ectoderm; the lateral 

 nerve-cords are mere thickenings of this sheath, and are 

 situated outside the layers of muscles : no stylet in the 

 proboscis. 



Ex. Carinella. 



Class II. MESONEMERTINI. 



A nervous sheath underlies the entire ectoderm the lateral 

 nerve-cords are more deeply situated, lying between the outer 

 circular and the inner longitudinal muscles : no stylet in the 

 proboscis. 



Ex. Cephalothrix. 



Class III. HETERONEMERTINI. 



A nervous sheath surrounds the body. Both it and the 

 nerve-cords lie inside a special layer of longitudinal muscles, 



