1873.]* 189 [McCrady. 



tory or depuratory office. Finally the hermapliroditism of Platyel- 

 minths finds itself repeated, though in a much less specialized form, 

 among the Ctenophorse. 



The Planarians, which seem among Radiata to be analogues of 

 Gasteropoda among Mollusca, though not parasites upon other ani- 

 mals, have a parasitism upon the earth involving habits wholly differ- 

 ent from those of the Ctenophorse, yet present a digestive system 

 eminently similar in type to that of Ctenophorse. Moreover, the 

 water system in Cestodes, and among Trematodes and Turbellarians, 

 both the digestive and water systems are, like the same systems in 

 Echinoderm larvae, Ctenophorse, and all other Acalephse, merely 

 hollowed out in the solid body. All these forms indeed, as well as the 

 Polypi, are Coelenterata. The body cavity of Echinoderms seems to 

 be a special modification of the right water-tube of the larva, and to 

 be itself* part of the water system. At any rate, such cavity here and 

 among the Nemertians and Nematelminths is a mere specialization, 

 and since, by general consent of naturalists, it presents no bar to the 

 association of these animals with the Coelenterate Planarians, it can 

 present none to an extension of Leukart's Coelenterata, so as to in- 

 clude in it all the Echinoderms, Gephyrians, Nematelminths and 

 Platyelminths, and so to bring us back to the Radiata of Cuvier, 

 emended of course by the elimination of Polyzoa, Rotatoria, pseudo- 

 helminths generally, and Infusoria. 



The existence of a vascular system sometimes witlj pulsatile en- 

 largements, among Echinoderms, Gephyrians, and Nemertians, is, like 

 the body cavity, a mere advance in specialization. The usually close 

 connection of this system, where it exists with the water system, 

 points to its interpretation as a specialization of that system; which 

 indeed, among Acalephs and Polyps, performs both functions. 



Similarly I regard the highly developed nervous system among 

 Gephyrians and Platyelminths as mere advances in specialization, 

 which like the entire apparatus of spines, hooks, exsertile proboscides 

 and highly developed muscular system, the external organs of gener- 

 ation and the existence of a true coitus, among the Helminths, must 

 be attributed to the requirements of their modes of parasitism. 



The Nematoda seem to be specialized upon the basis of the Cerca. 

 rian form. At least that interpretation is suggested by the frequently 

 lobed caudal extremity, which is sometimes bi-lobed or tri-lobed, as in 

 Gordius and Pseudalius; while Leptodera appendiculata and Filaria 

 appendiculata very strikingly recall Bucephalus in their caudal appen- 



