OF THE NEMATOIDS, PARASITIC AND FREE. 621 



partly admitting the difficulty of the different arrangement of the nervous system in 

 these two classes, he says, speaking of the ambulacral system in the Echinodenns and 

 the water-vascular system in the Scolecids, "It is impossible to compare these two 

 systems of vessels without bemg struck by their similarity ; each is a system of canals 

 opening externally and ciliated witliin, and the circumstance that the two apparatuses 

 are turned to different purposes in two distinct groups of the animal kingdom, seems to 

 me no more to militate against their homology, than the respiratory function of the 

 limbs of Phyllopod Crustacea militates against the homology of these limbs with the 

 purely locomotive appendages of other crustaceans." Uniting these two classes together 

 still under the name of Annuloida, Professor Huxley now thinks it would be better, 

 instead of retaining this as a division of the subkingdom Annulosa, to elevate it, in like 

 manner, to the rank of a " distinct primaiy division of the animal kingdom." 



Let us now turn our attention more particularly to the class Scolecida : in this Pro- 

 fessor IIuxLKY includes (provisionally rather than with any feeling of certainty) seven 

 groups, "the Motif era (or wheel animalcules), the Turhellaria* , the Trematoda (or 

 flukes), the Tceniada (or tape-worms), the Nematoidea (or thread-worms), the Acanthocc' 

 phala, and the Gordiacew" (p. 47). Of these, he seems pretty confident that the first 

 four groups have such a relationship as to demand their union in a single class on the 

 ground of a certain similarity in the arrangement of their nervous ganglia, and from the 

 fact of their all possessing that peculiar apparatus of vessels opening externally, which 

 has been called a " water-vascular system," whilst no heart, or vessels of any other kind 

 are known to exist. He imagines also that the system of reticulating canals beneath 

 the integument of the Acanthocepliala must be a modification of this apparatus. 



With regard to the Nematoids and their near allies the GordiacccB he is more doubtful, 

 though he says, " If the system of canals, in some cases contractile, which open exter- 

 nally near the anterior part of the body (fig. 22), and were originally observed by Von 

 SiEBOLD, and since by myself and others, are to be regarded as homologous with the 

 water-vessels of the Trematoda, this question must, I think, be answered in the affirma- 

 tivef . It is almost the only system of organs in the Nematoidea which gives us a defi- 

 nite zoological criterion, the condition of the nervous system in these animals being still, 

 notwithstanding the many inquiries which have been made into the subject, a matter of 

 great doubt." 



Our increased knowledge concerning the various modifications of the contractile canals 

 in the Nematoids, and the positive nature of our information relating to the nerAous 

 system of these animals, now place us in a much more faA-ourable position for consider- 

 ing the affinities of this particular group, and also tend, as I have already pointed out J, 

 not only to throw considerable light upon the functions of the water-vascular system, 

 but also to bind more intimately together the two classes Scolecida and EcMnodermata, 



* Including Nemertidce and Planarice. 



t As to whether the Nematoids should bo grouped with the four orders before mentioned. 



i Sec p. 603. 



