SWAINSON. 575 



so to speak, in existence," — wherein are contained animals that 

 have only a faint trace of affinity to those which, standing at 

 the head, exhibit the typical structure ; they possess merely 

 the rudiments "of the perfection to which they gradually, but 

 ultimately lead ; and their structure is often so exceedingly 

 simple, that in such as may be termed the germ, even those 

 rudiments are scarcely perceptible. One such, or at most 

 two, are all we can expect to find." — This sentence is intend- 

 ed to allure us to follow the author in his search for the 

 primary races of the Mollusca amongst the intestinal worms; 

 and he finds them there under the guise of slug-like para- 

 sites, and of other slug-like worms (Planariae), that have 

 their habitations in ponds and in the moist earth. " The 

 Parencliymata, or parasitic Mollusca, may be considered 

 the first indistinct and incipient development of the Tes- 

 tacea, — the point from which Nature diverges towards the 

 phytophagous Gasteropods on one side, and to the car- 

 nivorous Gasteropods on the other, until both these series 

 meet together, and form a perfect circle in the family of 

 Twbidce. It will subsequently appear that this remarkable 

 principle of variation is not merely confined to the first great 

 circle formed by the Testacea ; it is abundantly evident in 

 its primary divisions — nay, in some instances, even in its 

 families. Among the Cephalopoda it is particularly strong. 

 All writers who have mentioned the Foraminifora, so admir- 

 ably and beautifully investigated by D'Orbigny, hesitate 

 not to place these microscopic atoms in that order, although 

 it contains the most perfect Mollusca in existence : and yet 

 the organization 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 Cheli- 

 soma in the circle of the Dithyra, are further instances : 

 both are the most simple and slightly organized of their 

 separate groups ; and both, in this respect, as well as in the 

 shape of their bodies, are prototypes of the Planaridce and 

 Fasciolce, among the parasitic Testacea. But the univer- 

 sally 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, gela- 

 tinous, and so little organized, that, but for their indisput- 

 able and immediate affinity with the beautiful land shells of 

 the Helix 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 



