4< SHELLS AND SHELL-FISH. PART I. 



popular taste^ as to devote our present volume almost 

 entirely to so favourite a branch. This plan will enable 

 us to submit nearly the whole of our investigations to 

 the public ; while we trust to do the same hereafter in re- 

 gard to the remaining classes in a separate publication. 



(4.) The innumerable groups of beings which we 

 comprehend under the general name of Mollusca, are at 

 once distinguishable from all other animals^ by having no 

 internal bones, like the Vertebrata ; or no joints to their 

 body and limbs^ like the Annulosa, or insect tribes. In 

 the first, the skeleton is internal ; in the latter, it is ex- 

 ternal ; but, in the Mollusca, it is entirely wanting. 

 We might fill several pages on these anatomical dis- 

 tinctions ; but simplicity is the soul of instruction ; and 

 we feel assured, that in following this rule, in the pre- 

 sent instance, we are saving the unscientific reader a 

 world of perplexing circumlocution. 



(5.) We shall first glance at those characters drawn 

 from the nervous system, which, it is thought, sepa- 

 rate the Mollusca from the Vertebrata and the Annulosa. 

 On a former occasion*, we have stated that their nerv- 

 ous system is variable; and that this variation pointed 

 out the secondary divisions or classes, under which the 

 leading zoologists of the age have, almost unanimously, 

 arranged them. Thus, in the Acrita, or polypes, the 

 nervous system has been called granular; these granules, 

 innumerable and minute, being generally dispersed over 

 the bodies of these animals, which resemble '' masses of 

 transparent, homogeneous, mobile, and sensible pulp, 

 and thus impregnating the whole with sensibility ."t In 



* Classification of Quadrupeds, p. 37. 

 * f Such, at least, is~the definition of the Acrita of MacLeay; but our 

 recent researches among the Mollusca induce us strongly to doubt the 

 propriety of placing therein the greater part of those minute animals which 

 that celebrated naturalist has arranged with the corals. Our present im- 

 pression is, that the whole, or nearly so, of the true Acrita, are com- 

 pound zoophytes, or, in other words, plant-like animals ; that the Ro- 

 tifern and Infusoria are only prototypes of the Acrita in the circle of the 

 liadiata ; and that the annulose intestinal worms are the same among the 

 Annulosa. As for the Parcnchymata, it will be seen we have had no 

 hesitation in placing them as the representatives of this class in the most 

 aberrant order of the testaceous Mollusca. We feel, in short, more and 

 more persuaded that the real contents of the order Cyclobranchia cannot 

 be determined until the entire class of Acrita has been sufficiently ana- 



