458 WALTEli GAItSTANG. 



taken at the same time and from the same place as those 

 described by Koren and Danielssen. If I am right in this 

 supposition there is an exact correspondence between the 

 Plymouth and the Norwegian larvse in the number of ciliated 

 processes. The similarity between the two forms may be traced 

 even to details. Koren and Danielssen state that there are 

 three pairs of processes from the back of their larva, and this 

 is also the case in the Plymouth larva ; further, one of Miiller's 

 figures (1. c, Taf. iij fig. 1, Nos. 6 and 7) shows that the two 

 anterior dorsal processes arise from a common base on each 

 side, and they therefore represent a single bifid dorsal arm 

 similar to that described in the Plymouth larva. The paired 

 ciliated arms of the Plymouth form difi'er from those of the 

 Norwegian simply in their size and arrangement, being smaller 

 and less densely crowded together in the former than in the 

 latter. 



But in the case of the unpaired prse-oral arms there is a 

 marked difi*erence between the two forms. In the Norwegian 

 larva the dorsal arm is bifid at its end, and expanded into a 

 broad bilobed fin ; in the Plymouth larva it is broadly lanceo- 

 late, and tapers to a point at its extremity. This is the only 

 structural diff'erence between the two larvae, but its significance 

 cannot be definitely determined until it is possible to trace the 

 later development of the Plymouth larva. The much greater 

 elongation of the prse-oral lobe and the greater length of all the 

 paired ciliated appendages in the Norwegian form are corre- 

 lated with its mode of locomotion ; this has not been described 

 with much exactness, but it has been stated that during life 

 all the ciliated arms, both prae-oral and post-oral, are in a state 

 of incessant agitation. In the Plymouth larva, as already 

 mentioned, locomotion is carried on by means of the prae-oral 

 lobe alone. 



It is in this fact that the chief interest of the Plymouth 

 larva seems to me to reside. A great development and special- 

 isation of the prae-oral lobe for locomotive purposes is found 

 again in Balanoglossus, although all traces of ciliated bands 

 are lost in the course of its development from Tornaria. 



