3400 



MOSS ANIMALS AND LAMP SHELLS 



would hardly be enough to establish a relationship between the Brachiopods and the 

 worms. Their relationships rest rather upon the developmental history of the 

 Brachiopods and the transformations they undergo. The first stages in the develop- 

 ment of Theddium are briefly as follows: The developing eggs enter a pouch 

 formed by the lower mantle fold, into which also the two nearest arm fringes sink. 

 The latter become thicker, their ends swelling to form a pair of pads, round 

 which the eggs group, and to which each embryo is attached by means of a short 

 stalk. The embryo soon resembles a short thick annelid. The upper process 

 from its neck is the stalk by means of which the embryo is attached to the arm 

 fringe. The small anterior section resembles a head, and carries four eye spots 

 and a depression, the future mouth. There are two thicker middle segments, 

 followed by a fourth smaller segment, all covered with cilia. In the later 

 development the most posterior part is used for attachment, the head and collar- 

 like ring sink between the upward growing portions of the following ring. 

 These upward growths increase and form the two mantle folds. These, as has 

 been stated, secrete the shell. The illustration shows the young Theddium with- 

 drawn into itself, having given up the free-swimming life it led after breaking away 



from its parent. 



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 defenseless 

 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 organization, 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 



STAGES OK DEVELOPMENT OF about without causing the animal to close its valves. 

 Theddium (magnified). If some of the protruded cirri be touched, they are 



