IIKACIIlOHObA. 



315 



My derived from the ciliated ring visible in fig. 136*. The 

 ring of tentacles is placed obliquely, and the mouth is situated 

 near its ventral side. The tentacles appear to form a post-oral 

 circlet, like that of Phoronis (Actinotrocha): they gradually 

 increase in number as the larva grows older. 



Some of the later stages in the development 

 of the Terebratulidac have been made known to 

 us by the observations of Morse (No. 328 9) 

 on Tcrebratulina scptentrionalis. 



The most interesting point in Morse's observa- 

 tions 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 tentacles 

 begin to assume a horse-shoe arrangement, 

 which bears a striking, though probably acciden- 

 tal, resemblance to that of the tentacles on the 

 lophophore of the fresh-water Polyzoa. The 

 disc bearing the tentacles is prolonged anteriorly 

 into two processes, the free ends of the future 

 arms. By this change of shape in the disc the 

 tentacles form two rows, one on the anterior and 

 one on the posterior border of the disc, and 

 eventually become the cirri of the arms. The 

 mouth is placed between the two rows of tenta- 

 cles, 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 sinuations. 



Fn;. 138. DIAGRAM OF 



A LONGITUDINAL VERTICAL 

 SECTION OF AN ADVANCED 

 EMBRYO OF LlNGULA. (After 

 Brooks.) 



a. end of valves; b. thick- 

 ened margin of mantle; t. 

 mantle ; d. dorsal median 

 tentacle ; e. lophophore ; /. 

 lip ; g. mouth ; A. mantle 

 cavity ; i. body cavity ; *. 

 wall of oesophagus ; /. oeso- 

 phagus; m. hepatic cham- 

 ber of stomach ; n. intesti- 

 nal chamber of stomach ; o. 

 intestine ; ?. ventral gang- 

 lion ; r. posterior muscle ; 

 s. dorsal valve of shell ; /. 

 ventral valve of shell. 



1 In the abstract in Hoffman and Schwalbe Kowalevsky is made to state that the 

 tentacles spring from the border of the mantle. This can hardly be a correct account 

 of what he states, since it does not fit in with the adult anatomy of the parts. The 

 figures he gives might lead to the supposition that they sprang from the edge of the 

 cephalic lobe, or perhaps from the dorsal lobe of the mantle. 



