18 On the Form, Growth, and Construction of Shells, 



ON THE FORM, GROWTH, AND CONSTRUCTION 

 OF SHELLS. 



BY THE LATE DR. S. P. WOODWARD, F.G.S. 



Edited from his MSS. by Henry Woodward, F.G.S., F Z.S., of the British 



Museum. 



( Continued from 'page 253, Vol. x. November, 1866.) 



The Siphons. — As nearly all bivalves live buried in the sand 

 or mud, they are furnished with more or less elongated tubes, 

 one of which is called the inhalent and the other the exhalent 

 siphon (see Woodcuts, Figs. 10, 12, 14, and 15). 



Fig. 14. Tellina solidula (British). 



Fig. 15. Donax anatinus (British). 



Both having the foot protruded ; the arrows indicate the inhalent and 

 exhalent siphons. 



In those bivalve forms, like Mya, Lutraria, and Anatina, 

 in which the valves do not perfectly shut in the animal, the 

 siphons lie side by side, and are enclosed in the epidermis, 

 which is prolonged, and forms a strong horny envelope around 

 the shell and respiratory tubes. In certain boring and burrow- 

 ing bivalves, as Gastrochmna, Glavagella, and Teredo, the shell 

 does not increase with age, but the siphons secrete a shelly 

 tube in which the soft parts of the animal are incased, and the 

 minute valves of the young mollusk are seen embedded in the 

 wall (see Coloured Plate, Fig. 13, Vol. x., p. 241, the " water- 

 ing-pot shell," Aspergillum vaginiferum, the minute valves 

 are seen near the lower extremity of the tube). 



The External Ornamentation is another very apparent cause 

 of the immense variation in the form of shells. This is due 

 almost entirely to periodic growth. All mollusca, except the 

 Argonaut, possess a minute rudimcntal shell before they are 



