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at once withdrawn and the valves snap together, but 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 When the arms are withdrawn 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. mediterraneum, 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 
STAGES OF DEVELOPMENT OF Tlietidium 
(magnified). 
