I POSITION IN ANIMAL KINGDOM 5 



a position near to the Vertebrata. The relations of the Brachio- 

 poda 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 con- 

 siderable 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 re- 

 semblance. 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 Anne- 

 lida, 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.^ Each name, it will be noticed, bears refer- 

 ence 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. 



The G-asteropoda crawl on the flat under-surface or 'sole' of 



1 KecpaXri, head ; yaarrip, stomach ; a-KdirTecp, to dig ; ir4\eKvs, an axe ; ttovs, 

 7ro56s, a foot. 



