=Position of Mollusca in the Animal Kingdom.=--Up to very recent times it was usual to regard the Mollusca as one of the four subdivisions of a great family known as _Malacozoa_, the subdivisions being (1) Mollusca, (2) Tunicata, (3) Brachiopoda, (4) Polyzoa or Bryozoa. This classification is still retained in the leading modern manual on the subject.[3] The progress, however, of investigation leads to the belief that the Mollusca are not so closely related to these other groups as such a classification would seem to imply. The Tunicata, for instance, appear, from the whole course of their development, to occupy a position near to the Vertebrata. The relations of the Brachiopoda and Polyzoa will be more particularly referred to in that part of this History which deals especially with those groups. The position of the Mollusca is, in many respects, one of considerable isolation. Any attempt, therefore, definitely to relate them to one group or another, is, in all probability, to go further than the present state of our knowledge warrants. Especially to be deprecated are systems of classification which confidently _derive_ the Mollusca in general from this or that group. The first undisputed traces of animal life, which appear in the Cambrian epoch, exhibit the same phyletic distinctions as now exist. Sponges, Echinoderms, Mollusca, and Worms, formed already, in those immeasurably remote ages, groups apparently as generally distinct from one another as they are at the present time. It would seem that any theory of development, which confidently teaches the derivation of any one of these groups from any other, is, in the present state of the evidence before us, hazardous in the extreme. Some indications of relationship, which must not be pushed too far, may be drawn from a consideration of embryonic resemblance. An especial characteristic of the Mollusca is the possession of a particular form of larva, which occurs in one of the stages of development, known as the _trochosphere_ (see p. 130). This form of larva is shared with two orders of Annelida, the Chaetopoda and the Gephyrea armata, and, in all probability, with the Polyzoa as well. It may also be significant that the adult form in Rotifera bears a close resemblance to the trochosphere larva in those groups. =Basis of Classification.=--The Mollusca are divided into four great Orders--=Cephalopoda=, =Gasteropoda=, =Scaphopoda=, and =Pelecypoda=.[4] Each name, it will be noticed, bears reference to the ‘foot,’ _i.e._ to the organ of motion which corresponds in function to the foot in the Vertebrata. In the _Cephalopoda_ the feet, or, as they are more frequently termed, the ‘arms,’ are arranged symmetrically round the head or mouth. The common forms of ‘cuttle-fish’ (_Octopus_, _Loligo_) are familiar examples of Cephalopods.