HIGH SCHOOL ZOOLOGY. 237 



" sepia " that they diffuse around them for concealment). Al- 

 though fossil Dibranchiate forms are not uncommon ( Belem- 

 nites), yet the bulk of the fossil Cephalopods belong to the Tetra- 

 branchiata, the chambered shells of thousands of different species 

 furnishing to the Palaeontologist means of recognizing the rela- 

 tive age of the rocks in which they occur. 



MOLLUSCOIDEA. 



24. Two classes of animals require to be noticed in the present 

 chapter, which have been associated together as the sub-king- 

 dom Molluscoidea, partly on account of real, and partly on ac- 

 count of fancied affinities to each other and the Mollusca proper. 

 These are the Brachiopoda and Polyzoa ; the former, long con- 

 sidered to be related to the Lamellibi'anchs on account of the 

 possession of a bivalve shell ; the latter, on account of their 

 forming colonies, formerly classed with the " zoophytes " to be 

 described in the following chapter. There is no superficial re- 

 semblance between the two classes themselves, but zoologists 

 have determined by studying the development of both, that they 

 are not only related to each other, but also to the V^ermes. 



25. The Brachiopods are exclusively marine animals, compara- 

 tively few species of which survive to the present day ; in past 

 geological periods, however, they were extremely numerous, and 

 have, therefore, much of the same interest attaching to them as 

 the Trilobites and Tetrabrancbiate Cephalopods. Most of the 

 living species are found in the warmer seas ; of the few that 

 occur in the Gulf of St. Lawi-ence, Rhynchonella ^ysittacea is per- 

 haps the commonest. This species exhibits the characteristic 

 inequality of the two valves of the shell (Fig. 163), the 

 smaller (dorsal) valve fitting like a lid on the larger (ventral) 

 valve, which also has a projecting beak permitting the passage 

 of a short stalk by which the animal is attached. These valves 



