260 



ART 1C UL AT A. 



been made known to us by the observations of Morse (No. 328 9) on 

 Terebratulina septentrionalis. 



The most interesting point in Morse's observations on the later stages 

 is the description of the gradual conversion of the disc bearing the circlet 

 of tentacles into the arms of the adult. The tentacles, six in number, first 



form a ring round 

 the edge of a disc 

 springing from the 

 dorsal lobe of the 

 mantle ; in their 

 centre is the mouth. 

 In the later stages 

 calcareous spicula 

 become developed on 

 the tentacles. When 

 the embryo is far 

 advanced the tenta- 

 cles begin to assume 

 a horse-shoe arrange- 

 ment, which bears a 

 striking, though pro- 

 bably accidental, re- 

 semblance to that of 

 the tentacles on the 

 lophophore of the 

 fresh-water Polyzoa. 

 The disc bearing the 

 tentacles is prolong- 

 ed anteriorly into 

 two processes, the 

 free ends of the 

 future arms. By 

 this change of shape 

 in the disc the ten- 

 tacles form two rows, 

 one on the anterior 

 and one on the pos- 

 terior border of the 

 disc, and eventually 

 become the cirri of 

 the arms. The 

 mouth is placed between the. two rows of tentacles, where the two arms 

 of the lophophore meet behind. The position of the mouth was the original 

 centre of the ring of tentacles before they became pulled out into a horse- 

 shoe form. In front of the mouth is a lip. The arms grow greatly in 

 length in the adult Terebratulina. In Thecidium the oral disc retains the 

 horse-shoe form, while in Argiope the embryonic circular arrangement of 

 the tentacles is only interfered with by the appearance of marginal sinua- 

 tions. 



FIG. 137. Two STAGES IN THE DEVELOPMENT OF AHGIOPE, 



SHEWING THE FOLDS OF THE MANTLE GROWING OVEK THE CE- 

 PHALIC LOBE. (After Kowalevsky.) 



m. mantle fold; me. mesenteron; pd. peduncle; b. pro- 

 visional setse. 



The shell is deposited as two chitinous plates, which subsequently 



