CHAP. U. ON THE TYPES OF ACRITA. 45 



contains the most perfect Mollusca in existence. And 

 yet the organisation of these beings is so very simple, 

 that, if no regard be paid to the difference of analogy 

 and affinity, they might be placed next to the animalculae 

 in the class Acrita. The chitons among our Gasteropoda, 

 and the genus Chelisoma in the circle of the Dithyra, 

 are further instances : both are the most simple and 

 slightly organised of their separate groups ; and both, 

 in this respect, as well as in the shape of their bodies, 

 are prototypes of the Planaridw and Fasciolce, among 

 the parasitic Testacea. But the universally confessed 

 affinities of the naked slugs to the testaceous snails brings 

 this theory home to the personal cognisance of every 

 naturalist. Some of these creatures are so small, ge- 

 latinous, and so little organised, that, but for their in- 

 disputable and immediate affinity with the beautiful 

 land shells of the HelLv race, no one would think of 

 placing them in the same order, much less in the same 

 family ; and yet every zoologist sees that such is their 

 natural situation. Hence these naked slugs become 

 nothing more than prototypes of the Planarice and 

 Fasciolce ; related, indeed, to them by analogy, but 

 without any connection whatever in regard to affinity. 

 The extraordinary genus Herpa, one of the splendid 

 discoveries of Guilding (which will be subsequently 

 noticed), carries this analogical resemblance to the highest 

 pitch ; so that, but for the discriminating acumen of 

 that profound observer, we should have been tempted, 

 without seeing the animal, to consider it was an actual 

 type of Planaria, in the disguise, only, of a Liniaoc. On 

 these, and numerous other similar facts resulting from 

 the analysis of this class, we hesitate not to place a por- 

 tion of Cuvier's intestinal worms as the most aberrant 

 order of the testaceous Mollusca. We wish to follow 

 nature, and nature only ; perfectly satisfied, from past 

 experience, that, however various and conflicting may 

 be the comments of her interpreters, she has pursued but 

 one course, and but one plan, in her system of animal 

 variation. 



