456 WALTER GAHSTANG. 



but regularly repeated strokes of the dorsal arm of the prse-oral 

 lobe in an antero-posterior direction over the back of the larva. 

 Fig. 1 represents the position of the dorsal arm shortly before 

 the completion of a stroke. It is obvious that movements of 

 the broad flat dorsal arm in the direction mentioned must 

 propel the larva forward. The strokes of the arm occur with 

 wonderful constancy and regularity, except during short 

 intervals of rest ; I counted the number of strokes during three 

 consecutive minutes, and found them to be 80, 81, 81. The 

 dorsal arm of the prse-oral lobe is the only part of the body 

 engaged in this swimming movement. It might be imagined 

 that the ventral arm of the prse-oral appendage would also be 

 used for swimming, but I observed nothing to indicate that 

 such was in reality the case. The ventral arm and all the 

 paired processes of the body were entirely inert during loco- 

 motion. The ventral arm may perhaps be called into play at 

 a later stage of development, or its function may possibly be 

 simply that of counteracting any tendency to rotation ; iji 

 other words, it may take the part of an anterior rudder. 



3. Relations. 



It has been already mentioned that the Plymouth larva ex- 

 hibits points of resemblance to the common type of starfish 

 larva on the one hand, and to Sars's Bipinnaria asterigera 

 on the other ; it may, indeed, be regarded as a type intermediate 

 between these two forms. 



The body of the larva is quite normal, and the arrangement 

 of the ciliated bands difl'ers from that found in common 

 Bipinnarise only on account of the special development of 

 the prse-oral lobe. There is, however, a difference between the 

 two types of larvae in the form of the dorsal arms. The common 

 type possesses two pairs of dorsal processes, known in the 

 nomenclature of Agassiz as the dorsal oral and the dorsal anal 

 pairs of arms ; in the Plymouth larva three pairs of dorsal pro- 

 cesses are present. It will be seen, however, in fig. 1 that the 

 two anterior pairs of dorsal processes of the Plymouth larva 

 arise from a single base on each side, and that this common 



