138 HOMOLOGIES. 



isolated inner band of fibres. Proboscis has an external circular (or elastic), an internal 

 longitudinal, and a glandular layer supplied with acicular papillae. 



Genus XII. Cephalothrix, OErsted. — Head nearly cylindrical, slightly tapered in front ; eyeless, 

 or with a few obscure pigment-specks. Cephalic fissures and sacs absent. 



1. C. linearis, Jens Rathke. — Body extremely attenuate. Of a pale yellowish or skin-colour, 

 often with reddish grains towards the tip of the snout. 



VII. — Homologies. 



The majority of the early investigators of the Nemerteans correctly associated them with 

 the Planarians, and generally linked them to the Intestinal worms, Lumhrici, or Gordii, as a 

 single genus — Planaria. Other animals, however, which had no affinity either in form or 

 structure, were grouped with them, often in a perplexing manner. Lamarck thought the 

 Nemerteans approached the leeches, while Cuvier amalgamated them with his Entozoa. 

 Ehrenberg, again, while he took the wise step of forming a class {Turbellaria) for them and 

 the Planarians, does not seem to have had a very definite idea of their relationship to other 

 animals, and, more especially, to other Vermes. This author's class appears to me to be a very 

 natural one, and though a considerable hiatus exists between the Planarians and Nemerteans, 

 as will afterwards be pointed out, the gap is very much less than that which separates the 

 Turbellaria from the other groups of animals, and especially from the Trematoda. Delle Chiaje 

 considered they had certain homologies with the leeches, on account of the structure of the 

 " alimentary canal/' but that in regard to the form of their bodies they approached the Planarians. 

 Duges, De Quatrefages, and Frey and Leuckart, were inclined to link on the Turbellaria to the 

 Trematoda, though the second author was of opinion that further researches as to the vascular 

 system of the Planarians were needed to render the relationship distinct. In his report on the 

 memoirs of De Quatrefages, M. Milne-Edwards observed that the Nemerteans approached the 

 Annelids by the general disposition of their vascular system, the leeches by the structure of their 

 buccal system and other parts of their organization, but that their reproductive and digestive 

 organs were homologous with those of the helminths. He compared their nervous system to that 

 of the " Lingules." The statement with regard to the digestive system, however, is founded on 

 erroneous observations, since both reviewer and reviewed mistook the proboscis for the alimentary 

 canal, and thus instead of the latter forming a blind tube, it is open at both ends, and very 

 different from that of any helminth. (Ersted, again, placed them after the Leeches, while M. 

 Blanchard, misled by the observations of M. de Quatrefages, exaggerated the gulf between the 

 Nemerteans and the Planarians so much that he thought their affinities lay rather with the 

 helminths than with the latter. Dr. Thomas Williams considered his closed alimentary chamber 

 (digestive canal) the homologue of the spongy mass in Taenia, but this is open to doubt. He 



