178 Transactions of the Society. 



setiferous pistons, special muscles and nerves, throngli a succession of 

 shorter and simpler structures till they become mere pimples, or 

 even setiferous pits in the body-surface. The skin is hardened into 

 a perfect lorica in Braehionus ; is partially hardened in Dapidia ; is 

 merely tough in Mastigocerca ; and is soft and quite unarmed in 

 Notommata. The appendages of the body in Pedalion rise almost 

 to the dignity of crustaceous limbs, for they have joints, and are worked 

 by opposing pairs of muscles, passing across their cavities from point 

 to point. In Asplanchna these appendages become stumpy pro- 

 jections ; and the muscles, though still passing freely across the body- 

 cavity, are reduced to threads. In Triarthra the appendages become 

 chitinous spines ; and at last, when we reach Adineta, Taphrocamjoa, 

 and Alhertia, we find that we have passed from a Eotiferon closely 

 resembling a Nauplius larva, to one that is a simple worm. 



The internal structure is just as plastic. The characteristic trophi 

 exhibit a series of striking changes as we pass from one genus to 

 another. In one direction the change is due to the degradation of 

 the mallei ; in the other to that of the incus ; and in both this degra- 

 dation is pushed so far that the changing parts may be said almost to 

 disappear. For in Braehionus and Euchlanis the mallei are well 

 developed ; in Furcularia mere needle-shaped curved rods ; in 

 Asplanchna so evanescent that it is hardly possible to find them in an 

 animal killed by pressure. 



By another set of changes the rami are, in their turn, reduced 

 almost to evanescence, becoming feeble loops in Stephanoceros, and in 

 Floscularia two membranes attached to the unci. 



Changes, great in degree, if not in variety, occur also in the 

 excreto-respiratory system. For the contractile vesicle, which fills 

 quite half the body-cavity in some Asplanchnee, dwindles down in 

 various species till it seems to vanish in Pterodina and Pedalion; 

 while in one abnormal form, Trochosphsera, the connection between 

 the lateral canals and the contractile vesicle is snapped, and the latter 

 becomes an appendage of the cloaca only. 



The nervous system, wherever it has been made out, is indeed 

 always on the same plan ; but its central organ, the nervous ganglion, 

 is in Copeus and Euchlanis a great cylindrical sac, stretching from 

 the head below the mastax ; while in Floscularia it shrinks into a 

 email star-shaped body between the eyes and the organ of taste. 



The alimentary and reproductive systems are those which 

 vary the least ; but even here the difference in proportionate size is 

 very great between the stomachs of Sacculus and Synchseta ; and 

 also between the ovaries of Asplanchnopus myrmeleo and Asplanchna 

 priodonta. 



But not only do most of the external parts and internal organs 

 vary in turn almost to vanishing, but these variations are not in any 

 way simultaneous. The result is, that we find an organ, of a form 

 characteristic of one family or genus, occurring in a species that belongs 

 to another. 



