THE LAMP SHELLS 



340i 



at once withdrawn and the valves snap together, but soon open again. When the 

 arms are withdrawn the cirri are bent inward, but when the valves open the former 

 are seen to raise themselves into an upright position; even before the shell was 

 opened a few cirri were often protruded and waved to and fro, as if to ascertain 

 whether any danger threatened. An inflowing current of water can sometimes 

 be observed between the two rows of cirri. 



Another form, Waldheimia cranium, is found near the North Cape, at a 

 depth of from twenty-five to one hundred and fifty fathoms, attached to stones or 

 barnacles. The calcareous framework in Waldheimia is long, and the oral append- 

 ages are incapable of movement unless it be at their spirally-coiled ends. It has 

 been conjectured that the two coiled ends can be unrolled and rolled up again like 

 the proboscis of a butterfly. These animals are more active than Terebratulina, 

 frequently moving about on the ends of their stalks and being more easily alarmed. 

 The cirri do not project beyond the edge of the shell, and are bent back when it is 

 closed. The genus Thecidium, whose development has been described, is distin- 

 guished by the very peculiar calcareous framework of its arms; one of its few living 

 representatives being the unstalked T. mediterraneum, figured below. In this 

 form the dorsal valve forms an almost flat lid for the much larger ventral valve, and 

 is seen in the figure standing wide open at right angles to the lower shell. The 

 calcareous framework nowhere rises freely from the lid, with which it is connected 

 by a calcareous network. In the section (Z?) given in the illustration we see in the 

 dorsal valve the depression for the hinge on which the valve rotates. The shell is 

 opened by the muscles () which run from the bottom of the ventral valve to a 

 process directed backward in the dorsal valve behind the hinge. It is closed by 

 the muscles marked a, which lie in front of the hinge. The shells of Thecidium. 

 become attached to submarine objects, and 

 are brought up in considerable numbers by 

 the nets of the coral fishers between the 

 Gulf of Bona and Cape Rosa, from a depth 

 of from forty to fifty fathoms. The number 

 of specimens of Terebratula is small as 

 compared with that of Thecidium, twenty 

 to thirty specimens of the latter being 

 often found together. When first caught, 

 Thecidium opens its valves very wide, but 

 when isolated and placed in small vessels 

 gapes less widely. The small dorsal valve 

 or lid can be raised to form a right angle 

 with the other valve, but, when the slightest 

 movement is made, it snaps to with the 

 speed of lightning. These lamp shells are 

 undoubtedly sensitive to light, even a 



shadow thrown upon them making them close their shells instantly. On account 

 of the wide gape, the inner organs, such as the cirri and arms, can be accurately 

 observed. The inner surface of the shell on which the mantle lies is so dazzlingly 



Thecidium mediterraneum. A. Natural 

 size; B. Section through the shell 

 (magnified) . 



