8 MANUAL OF THE MOLLUSCA. 



special breathing organ, but the mantle performs that office ; they 

 take their name from two long ciliated arms, developed from the 

 sides of the mouth, with which they create currents that bring 

 them food. 



Fig. 4, 5, 6. Bmchiopoda* 



5. The coricUfera,-] or ordinary bivalves, (like the oyster), 

 breathe by two pairs of gills, in the form of flat membraneous 

 plates, attached to the mantle ; one valve is applied to the right, 

 the other to the left side of the body. 



6. The tunicata have no shell, but are protected by an elas- 

 tic, gelatinous tunic, with two orifices ; the breathing-organ 

 takes the form of an inner tunic, or of a riband stretched across 

 the internal cavity. 



!Five of these modifications of the molluscan type of organiza- 

 tion, were known to Linnaeus, who referred the animals of all 

 his genera of shell-fish to one or other of them ; J but unfortu- 

 nately he did not himself adopt the truth which he was the first 



* Fig. 4. (3). Rhynchonella psittacea,C\\em, sp., dorsal valve, with the 

 animal (after Owen). 5, 6, Terebratula australis, Quoy. From specimens 

 collected by Mr. Jukes. (2). Ideal side view of both valves, (f, the retractor 

 muscles, by which the valves are opened). (1). Dorsal valve. These wood- 

 cuts have been kindly lent by INIr. J. E. Gray. 



f ConcJdfera, Shell-bearers. 



:j: The Linusean types were — Sepia, Limax, Clio, Anemia, Ascidia. Tere- 

 hratula was included with Anomia, its organization being unknown. 



