CHAPTER VIII. 



THE P0LYZ0A, THE BRACHIOPODA, AXD THE MOLLUSCA. 



However diverse in outward appearance and in com- 

 plexity of organization the multitudinous forms of animals 

 which have been described in the preceding four chapters 

 (Chap. IV. to VII.) may be, the student passes from one to 

 the other, by easy and natural gradations, from the simple 

 Turbeilarian at the bottom to the most highly differentiated 

 Arthropod at the summit of the series. But with the higher 

 Crustacea, Araehnida, and Lisecta the scale ends ; from none 

 of these are we led to any higher form of animal life. 



The Cuttle-fish, the Whelk, the Snail, and the other in- 

 numerable forms of animals w T ith univalve, bivalve, and mulr 

 tivalve shells, which are commonly known as 3follusca, are 

 so widely different, not only from the Arthropoda, but from 

 all the higher members of the group of Worms (Chap. V.), 

 that any connection with these seems, at first, to be wanting. 

 The segmentation of the body, which is so conspicuous a fea- 

 ture of the greater number of the members of the series 

 which ends with the Arthropods, is absent ; limbs are want- 

 ing ; instead of the equality .of the neural and haemal faces 

 of the bilaterally symmetrical body, and the consequent 

 remoteness of the oral and anal apertures, which is usual 

 among the Arthropods and Worms, these two faces are usual- 

 ly unequal. The haemal face is often produced into a longer 

 or shorter cone ; the anus is, as a rule, approximated to the 

 mouth ; and, very often, the haemal face of the body is asym- 

 metrical. 



The higher Mollusks, in fact, form the final term of a 

 series of their own, which commences in the Polyzoa, with 

 animals which have many resemblances to the Rotifera. 



The Polyzoa or Brtozoa. — In outward form these ani- 

 mals bear a general likeness to the Sertularian Hydrozoa^ 



