OF THE NEMATOIDS, PARASITIC AND FEEE. 625 



existing in the Echinddeims ; whilst the other variety includes an indefinite number of 

 much smaller intcgumental channels, Avhich, as far as function goes, are in all proba- 

 bility also respiiatory and may be analogous to tlie second variety, or antambulacral 

 pores in the Echinoderms, though in their modes of distribution they present many 

 resemblances to the different modifications in the arrangement of the first or ambulacral 

 series. Seeing that the apparatus in the Nematoids supposed to be homologous with 

 the ambulacral vessels of the Echinoderms subserves an entirely difiercnt function, and 

 that the locomotive feet are no longer required, we can well imagine that the pores 

 de^■elopcd in connexion with them in the Echinoderms would be luirepresented in the 

 Nematoids. Either, then, the pores existing in the Nematoids may be considered as the 

 homologues of the ambulacral pores, with which they exhibit so close a correspondence 

 in the method of their distribution though subservient to a different function, or, these 

 being absent altogether, we may suppose their vacant place and method of distribution 

 (so characteristic of the Echinoderm type) to have been appropriated in the Nematoids 

 by the representatives of the respiratory antambulacral pores of the Echinoderms. The 

 arrangement of the ambulacral pores in the Echinoderms is generally along lines 

 radiating from the mouth ; and though five, or multiples of five, are the prevailing 

 numbers met with amongst these animals, still in the family Spatangacew the ambulacral 

 pores are distributed along four double lines instead of five, whilst in some Ilolotlmnce 

 they arc confined to one aspect of the body, and in others are scattered irregularly 

 over its whole surface. So amongst the Nematoids the integumental pores may be 

 found in some genera, such as Oncliolaimus and Leptosomatum, along four longitudinal 

 lines radiating from the mouth; in Dorylaimus and others along two; in Trichosoina* 

 along one, two, or three; in Trichocephalus along one Avidening band, and in such 

 numbers as infinitely to exceed the 3720 ambulacral pores calculated by Professor Foebes 

 to exist in the Common Egg-urchin; whilst in Ebkrtii's Enoplus ca'Tuleus'^ they would 

 appear to be more sparingly scattered over the whole surface of the body. 



Surely there must be some homologies between these organs existing in the Echino- 

 dermata and Nematoidea respectively. And if not, where else are we to look amongst 

 the lower invertebrata for the representatives of these integumental pores which have 

 been shown to be so numerous in the free and in some of the parasitic Nematoids % 



All the Nematoids and almost all the Echinoderms are dicccious ; and though in the 

 majority of the latter the orifices of the genital organs are multiple, still in the IIolO' 

 tlmriadce there is only a single exit in the dorsal region of the body. As pointed out by 

 MtJLLEE, the Echinoderms never exhibit a perfect radial symmetry, and in some whole 

 families even, such as the Spatangoids and Clypeastroids, there is an approximation 

 to a bilateral symmetry ; whilst the fact is as certain as it is remarkable, that " in the 

 larval state * * radial symmetry is totally absent, the young Echinoderm exhibiting as 

 complete a bilateral symmetry as annelids or insects" J. The peculiar method by which 



* Eberth, TJntersuch. uber Nemat. Taf. vi. u. vii. t Loc. cit. Taf. vii. 7t 



J Huxley, Elements, p. 47. 

 MDCCCLXVI. 4 R 



