412 



INVEETEBEATA 



CHAP. 



oesophageal ganglion. A quite similar sense -plate is formed in 

 the mid -ventral line behind the rudiment of the stomodaeum. 

 Whether this plate bore cilia or not was not determined, but its 

 cells are the same shape as those of the apical plate, and, from their 

 bases, ganglion cells are cut off which are the rudiment of the 

 important sub-oesophageal ganglion of the adult (Fig. 326). 



Before this stage has been reached the embryo has become trans- 

 formed into a free-swimming larva of most characteristic form. The 

 body is divided by constrictions into three segments. Of these the 

 anterior, or head segment, is conical and ciliated all over like the hood 

 of the Actinotrocha larva, but carries in addition an apical tuft of 

 long cilia ; the second, or mantle segment, carries two long backwardly 



FIG. 326. Frontal and sagittal sections of the larva of Terebratulina septentrionalis. 



(After Conkliu.) 



Letters as in the two preceding figures. In addition, coe.m, extension of coelom into mantle-fold ; 

 sb.o.g, rudiment of suboesopliageal ganglion. 



directed folds, a dorsal and a ventral, both of which are ciliated ; the 

 dorsal carries the bundles of chaetae on its edge ; the third, or foot 

 segment, carries no cilia or chaetae. Since the larva possesses no 

 mouth, and is therefore unable to feed itself, its free life must be of 

 short duration. 



For the account of its metamorphosis we are entirely indebted to 

 Morse, and, as he did not use the method of sections, we know only 

 the external features of this period of the development of the larva. 

 After swimming for some time, not in any case longer than twenty- 

 four hours the larva fixes itself to a suitable substratum by the end of 

 the foot. This " foot " becomes transformed into the peduncle or stalk 

 of the adult. Then the mantle-folds lose their ciliated covering, first 

 on the ventral and then on the dorsal side (Fig. 327) ; then they are 

 suddenly retroverted so as to project forwards instead of backwards, 



