LAMP-SHELLS. 



429 





The developmental transformations of another genus, Argiope, are very 

 instructive. Its larva may not only be compared with that of a bristle-worm- 

 it is in reality such a larva. No further develop- 

 ment, however, occurs in this direction, but rather 

 a degeneration. It becomes transformed into a 

 creature which has no resemblance to an annelid. 

 The posterior end changes into a stalk, by means of 

 which the animal is permanently attached, while 

 the bivalve shell protects the otherwise defenceless 

 body. In this case we can witness the degeneration 

 of an animal in its own development. It begins as 

 if it were going to be a highly developed worm, 

 which seems to show that its ancestors were once 

 such worms, but it disappoints us ; instead of 

 advancing in organisation, it suddenly drops back 

 into the lowly creature described. Off the coast of 

 Norway, the serpent-headed Terebratulina is found 

 everywhere in small numbers, at a depth of from 

 thirty to one hundred and fifty fathoms, often 

 attached to the coral Oculina. When placed in sea- 

 water, they gradually open their valves ; those 

 specimens which remain attached to foreign objects 

 show a great disposition to move about at the ends 

 of their stalks. Detached specimens can be moved 

 about without causing the animal to close its valves. 

 If some of the protruded cirri be touched, they are 

 at once withdrawn and the valves snap together, but 

 soon open again. When the arms are withdrawn 

 the cirri are bent inwards, 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 appen- 

 dages 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 is described above, is distinguished 

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

 sentatives being the unstalked T. mediterranewm, figured on p. 430. 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 



,0. 





STAGES OF DEVELOPMENT OF TllCcilUinn 



(magnified). 



