Phylogeny of Animals 527 



line." The above accounts for rotifers and lamellibranchs 

 seem to agree so well that additional evidence either of nega- 

 tive or confirmatory kind is highly desirable. 



Though the resemblances may be only structural parallel- 

 isms and trivial, it might further be pointed out that a striking 

 similarity seems to exist between the circlets of — in part cili- 

 ated — plates in the primitive genera Yoldia and Nucula, and 

 the tesselated plates of the rotifers Aruircea and Metopidium. 



So the dorsal shell or prodeltidium of brachiopods, the single 

 larval shell of gasteropods, and the dorsal larval shell of lamel- 

 libranchs, seem all traceable, as primitive structures, back 

 to the dorsal lorica of rotifers, while in the last the first faint 

 beginnings of the molluscan shell-gland seem strongly sug- 

 gested by Gosse's accounts for the rotifers. 



The trochal or velar area in larval lamellibranchs gives rise 

 to the two pairs of highly sensitive labial palps. These have 

 been regarded by Loven (i7i: 46) as mere expansions of the 

 velum. But their position in relation to the head and larval 

 eyes, as well as their innervation, would suggest rather that 

 they are the four rotiferan antennae in soft expanded and some- 

 what modified form. 



The musculature of bivalve shells is so conspicuous a feature 

 that some commencing indication of it might be looked for 

 in rotifers. An observation of Hudson regarding Pterodina 

 valvata is worth quoting. He says: "While watching some 

 of the new Pterodinae, I was surprised to see one of them sail- 

 ing by, with its lorica folded down like the flaps of a Pembroke 

 table; its outline was so altered that it scarcely seemed the 

 same animal. This curious infolding of the lorica is due to 

 the contraction of two conspicuous transverse muscles, which 

 do not necessarily act together; as a friend, who was watching 

 with me, saw some specimens with only one side folded at a 

 time" {169,11: 113). 



As to the main longitudinal muscular bundles, the striking 

 resemblance in disposition, insertion, number, and function 

 of those seen in Plate XI, fig. 3, of the above work, for As- 

 planchna, or the description on page 61 for Stephanoceros with 

 those of Teredo larva as figured from Hatschek, may be a case 

 of parallel coincidence. But in view of the many other funda- 

 mental points of agreement, we can scarcely view such other- 

 wise than as a case of adult rotiferan and larval molluscan 

 continuity. 



The foot exhibits equally great variation in size and func- 

 tion in both groups. Thus it may be elongated, tapered, and 

 attaching quickly at will, or greatly elongated, muscular, and 



