404 THE ANATOMY OF INVERTEBRATED ANIMALS. 



extending beyond the lias, while the majority of the species 

 of the other two families are also extinct. 



The family of the TerebratuUdce, which is not certainly 

 known to occur in formations older than the Devonian, is the 

 only one in which, since the end of the palaeozoic epoch, 

 numerous new generic types appear. 1 



The Inarticulata have no hinge ; the intestine opens into 

 the cavity of the mantle, the margins of the lobes of which 

 are completely separate. Some have a long peduncle (Lin- 

 gula), others are fixed by a plug which passes through an 

 aperture or notch of one valve (Discina), or by the surface 

 of one valve ( Crania). There is no brachial skeleton, and the 

 arrangement of the muscles is in many respects different 

 from that which obtains in the articulated division. 



Species of all these families, except the Spiriferidoe, 

 Orthidw, and Productidce, exist at the present day, but they 

 are also represented in the older palaeozoic epochs, and Lin- 

 gulee are among the oldest known fossils. 2 



The Mollusca. — The term Mollusca may be used as 

 a convenient denomination for the Lamellibranchiata and 

 Odontophora (= Gasteropoda, Pteropoda, and Cephalopoda, 

 of Cuvier), which can be readily shown to be modifications of 

 one fundamental plan of structure. This may be represented 

 by a body, symmetrical in relation to a median vertical plane, 

 at one end of which is the oral and at the other the anal 

 aperture of the alimentary canal. In the body a ventral, or 

 neural, face, an opposite dorsal, or hmmal, face, and a right 

 and left side may be distinguished. The neural face usually 

 gives rise to a muscular foot. The integument of the haemal 

 face is generally produced at its edges into a free fold, and 

 the term mantle, or pallium, is applied to the region of the 

 integument thus circumscribed. Between the free portion of 

 the mantle and the rest of the body is a cavity, the pallial 

 chamber, from the walls of which, processes which subserve 

 respiration, the branchial, may be developed. 



In the median line of the surface of the mantle of the em- 

 bryo a shell-gland is very generally formed, and from the 

 surface of the mantle a cuticular secretion, the shell, is pro- 

 duced. 



1 Suess, " Ueber die "Wbhnsitze der Brachiopoden." (" Sitzb. d. "Wiener 

 Akad.," 1857.) 



2 See Davidson's " Monographs of British Fossil Brachiopoda," in the Pa- 

 lceontographical Society's publications. 



