MORPHOLOGICAL METHOD AKD RECENT PROGRESS IN ZOOLOGY. 599 



sion of recurrently ciliatecl zones, which by reduction, with union and 

 translocation forwards,' give rise to the trochal lol)e. It is now known 

 that in th-e American pelccypod Vol did Jinuttula a siniihir stage is 

 found, in v.diich a ''test'' of tiA'e rows of ciliated cells is present; and 

 of the young of I><ni<l< rxhi Ixi mjuJcuxix i\w like is true. But whereas 

 in the Yold'id the ciliated sac is ultimately shed, in the Myzomenian 

 the escape of the eml^ryo is accompanied ))y rupture, which liberates 

 the anterior series of ciliated zones in a manner strongly suggestive of 

 forward concentration, Itniving the posterior circlet with its cilia 

 attached. 



This "test'' has also been seen in two species of ±{vrnhi^ and pend- 

 ing fuller in(|uiiy into the Myzomenian and a reinvestigation of Den- 

 talium, I would suggest that this recurrently ciliated sac is represent- 

 ative of a larval stage antecedent to the trocliophore, for which the 

 term protrochal may suffice. This term lias indeed been already 

 applicnl to a larva of certain Polycha'ta, which might well represent 

 a modification of that for which J am arguing; and quite recent!}' it 

 ai)pears to have been observed near Cevlon for a species of the genus 

 2Ln'pJnjs<(. 



The discovery of this larva in Dondersia was accompanied hy that 

 of a later-formed series of dorsal spiculai" })lates, which for once and 

 for all, in realizing a chitonid stage, demolish the heresy of the 

 " Solenogastres," mischi(>vous as suggesting an affinity with the worms. 

 Like that of the supposed cephalopod affinities of the so-called " Pter- 

 opods," it must be ignored as an error of the past. 



Returning to the protrochal stage, whatever the future may reveal 

 concerning it, by bringing together the Lamellibranchiata, Scaphopoda, 

 and Polyplacophora, it associates in one natural series all the l)ilater- 

 ally symmetrical Mollusca except the cephalopods. In doing this, it 

 deals the death l)low to the supposed Rhipidoglossan affinity of the 

 Lamellibranchiata; and in support of this conclusion I woidd point 

 out that the recently discovered eyes of the mytilids are in the posi- 

 tion of those of the embryo CJiJtoii, and that just as J)<'/daJhiiii, in tiie 

 formation of its mantle, passes through a lamellibranchiate stage, so 

 are there lamelli])ranchs in munber in which a tubular investment is 

 found. 



This protrochal larva has an important part to play. It may very 

 possibly explain phenomena such as the compound nature of the tro- 

 chal lobe of the limpet, the presence of a post-oral ciliated band in the 

 larva of the shipworm, and of a pra'-anal one in that of various mol- 

 luscan forms. In vic^w of it, we must hesitate before we fully accept 

 the belief in the ancestral significance of the trochophore. And it is 

 certain that an idea, at one time entei'taiiu^d, that the Rotifer {Tro- 

 chosp/Ki'rd), which so clos<dy rescMubh's it as to bear its name, is its 

 persistent repVesentati\(\ is wiong, since this is now known to bo but 

 the female of a species ha\ ing a \ cry ordinary male. 



