574 



INVERTEBEATA 



CHAP. 



Heider was able to keep the larvae living for eight days. Though 

 still ciliated all over, a concentration of ciUated cells was observable 

 along certain hues corresponding to the position of the ciliated bands 

 of the full-grown Tornaria larva ; these are a longitudinal folded 

 band with a marked backwardlyi directed prae-oral loop, in all respects 

 similar to the main cihated band of a young Bipinnaria larva, and a 

 posterior transverse cihated band which is the main external feature 

 which distinguishes the Tornaria larva from the young Bipinnaria 



and Auricularia larvae. This band 

 corresponds roughly in position 

 to the telotroch of Annelid larvae 

 and will receive the same name, 

 viz. telotroch {ttr, Fig. 418). The 

 apical plate laad by this time 

 developed two eye-spots which 

 were simple cups of ectoderm cells 

 surrounded by pigment. The plate 

 is situated just at the spot where 

 the prae-oral loop originates from 

 the main part of the longitudinal 

 cihated band. The anterior coelom 

 now sent out two posterior spurs 

 which arched round the oesophagus, 

 and in the oldest larvae Heider 

 was able to detect the origin of a 

 pair of posterior coelomic vesicles 

 which form the rudiment of the 

 trunk -coelom. These arose as 

 evaginations, with sht-Hke lumina, 

 of the anterior wall of the intestine, 

 just behind the groove which 

 marks it off from the stomach. 



Morgan's observations (1891) 

 on the development of the New 

 England Tornaria commence 

 just at the stage where Heider's 

 observations leave off. His material consisted of a swarm of 

 Tornaria larvae of all ages which were caught by the tow-net off 

 Wood's Hole, Massachusetts, in the summer of 1890. The youngest 

 of these were only \ mm. long, but in them the anterior coelom was 

 entirely separated from the gut and the proboscis-pore had been 

 formed. The two ciliated bands were distinct, but the anterior one 

 was barely folded. The posterior coelomic sacs had not yet been 

 formed. . The apical plate was completely fused with the sides of the 

 longitudinal band at the prae-oral pole of the larva, where these sides 

 approach most closely to one another ; it thus formed a bridge 

 delimiting a prae-oral loop from the rest of the band. On the apical 

 plate were two eye-cups, an anterior and posterior. These were 



Fig. 418. — Surface view of the young 

 Tornaria larva of Balanoglossus clavi- 

 gerus, four ilays old. ( After Heider. ) 



a, anus ; ay, apical plate ; cil.loTig, longitudinal 

 band of cilia ; o, moutli ; oc, eye-spots ; ttr, 

 telotroch, i.e. posterior transverse band of cilia. 



