v PARASITIC FORMS AFFINITIES I 19 



Parasitic Forms. The genus Malacobdella was found by 

 von Kennel 1 in large numbers living on Cyprina islandica, 

 a Lamellibranch Mollusc, in the harbour at Kiel ; and it 

 has also been described by Kiches 2 as a British form. It 

 is attached to its host by means of a large round sucker 

 situated at the posterior end of the ventral surface, while 

 the rest of the body waves about freely in the mantle-cavity. 

 It is perhaps hardly correct to describe this animal as para- 

 sitic, since it does not appear to obtain its nutriment 

 at the expense of the host by preying on its juices. The 

 advantage of its position is, however, 

 obvious, since a perpetual current of 

 water is kept up in the mantle -cavity 

 of the Mollusc, and from the stream the 

 Nemertine is able to pick out and take 

 for itself any food material which it 

 considers suitable. At the same time 

 it is not subjected to the influence of the ^ 



J FIG. 61. Matacobddla grossa 



winds and waves, as the shell of the o. F. Mull., a large female 

 mollusc acts as a barrier to prevent the (From 16 von Kranei ) X A 

 entrance of disturbing elements. From the dorsal surface ; B, 



Malacobdella is short and broad, some- from tbe veutral surface ' 

 what flattened dorso-ventrally. The anterior end is bluntly 

 rounded. The mouth opens into a wide pharynx, which is con- 

 stricted behind and then passes into the intestine ; this after a 

 few coils opens by the anus situated dorsally immediately above 

 the sucker. The proboscis opens into the pharynx. 



Palaeontology. Nemertines are unknown in a fossil state ; 

 this is probably owing to the softness of their bodies, which would 

 render their preservation extremely improbable. 



Affinities. Until recently the Nemertines were regarded as 

 a sub-order of the Turbellaria. They were afterwards separated 

 from the Turbellaria and placed as a distinct class of the phylum 

 Platyhelminthes. 



Some zoologists have considered them to be so different in 

 many respects from the other classes of the Platyhelminthes as to 

 justify their being altogether separated from that phylum, and 

 treated as a distinct group. 



1 Art. Inst. Wiirzburg, Bd. iv. 1877-1878, p. 305. 

 - Jvurn. Mar. L'iol. Ass. vol. iii. 1893-1895, p. 22. 



