SOME BIPINNARItE FHOM THE ENGLISH CHANNEL. 458 



oral lobe, but is continued backwards down each side of the 

 body to the posterior end, where it bends first ventrally, then 

 anteriorly, to pass between mouth and anus across the mid- 

 ventral region of the body. The mouth is encircled by its own 

 ad oral band (fig. 2, a. b.). The anterior or prae-oral ciliated 

 ridge (cephalotroch of Lankester) is produced into three pro- 

 cesses — a median arm anteriorly {p. v. «.) and a pair of lateral 

 arms posteriorly (r. a. v. a. and counterpart). The posterior 

 or post-oral ciliated ridge (branchiotroch of Lankester) is pro- 

 duced into eleven processes — a median arm anteriorly {p. d. a.), 

 and five pairs of lateral processes. Of the lateral processes 

 three pairs are dorsal, one pair is posterior (r. p. I. a. and 

 I. p. I. a.), and one pair is ventral in position {r. p. v. a. and 

 counterpart) . 



Each of the anterior unpaired arms arises from the apex of 

 a prolongation of the prse-oral lobe, which may be termed the 

 prse-oral appendage (cf. fig. 1). This appendage possesses a 

 very characteristic form. It is broad from side to side and 

 compressed dorso-ventrally, the degree of compression in- 

 creasing with the age of the larva. Down each side of the 

 appendage runs a groove which is continuous with the lateral 

 depressions of the body of the larva, and separates the anterior 

 prolongations of the prse-oral and post- oral ciliated bands from 

 each other. The prse-oral appendage bifurcates after a certain 

 distance into a dorsal and a ventral part, the two anterior un- 

 paired ciliated arms, of which the dorsal is the larger. The 

 point of bifurcation (fig. 1, w.), where the prse-oral and post- 

 oral ciliated bands also diverge from each other, represents the 

 morphological apex of the prse-oral lobe. This is clearly shown 

 by a comparison of the larva with Tornaria; it is at this spot 

 — between the anterior extremities of the two ciliated bands — 

 that the apical plate with its tuft of cilia and pair of pigmented 

 sensory pits is situated in the larva of Balanoglossus. The 

 dorsal and ventral prse-oral arms {p. d. a., p. v. a.) are broad 

 and flattened, and have lancet-like or tongue-shaped termina- 

 tions. The dorsal arm bends backwards over the dorsal side 

 of the body, while the ventral arm has an equally pronounced 



