572 THE POPULAR SCIENCE MONTHLY, 



a bivalve box, or case, to inclose the creature as snugly as you see 

 effected for a clam or an oyster. Loss of head and tail would leave 

 the little cypris as perfectly a testacean moUusk as any of these. 



But you say, if — if it should lose head and tail ! The exaggerated 

 fable of the fairy tale is actually the method adopted by the magician 

 Nature to disenchant the fair princess Life from the thraldom of lar- 

 val and infusorial forms. The cypris does lose head and tail, and so 

 becomes molluscan, before proceeding to higher evolution. But not 

 as a cypris — oh, no! slie changes her name, as she changes her type; 

 and we must advance to another group of the entomostracous class — 

 one which has been divided from it, more on account of general supe- 

 rior size than for any other reason — the cirrhipeds. 



In this group we find the most striking metamorphoses in the life 

 of the same individual. Passed out of the mere embryo, the larval 

 form resembles more an ordinary macrourous decapod than the ento- 

 mostracous its nearest relations. Take one generally known, the ge- 

 nus Upas — the common goose-barnacle — in extreme youth, an exceed- 

 ingly active little shrimp-like tenant of the deep. During the latter 

 part of this stage, the hypertrophied segments of the thorax continu- 

 ing to grow as the cephalic and abdominal parts dwindle, finally, by 

 his jaws, he first seizes hold of some solid support, when the whole 

 head and neck become entirely changed, and remain a mere stipe, for 

 the support of the creature. Every part becomes metamorphosed. 

 The head having disappeared, a new mouth is opened in the breast; 

 and the abdominal portions, although not entirely lost in this genus, 

 are differentiated to other functions. The most important change to 

 notice, in this creature of change, is the change of axis. It is now ex- 

 actly at right angles to the original axis of vitality in the young crus- 

 tacean. Yet, mark well: this is only a return to the true axis of nerve- 

 force in the segments, which, in all annulosa^ is at right angles to the 

 longitudinal development of the sections. The interminable gemma- 

 tions and addition of segments to the zoary being arrested, and life 

 confined to one or two, most naturally the current of the dominant force 

 remaining controls the direction or axis of all the rest. So completely 

 have the method of vitality and the appearances of the little animal 

 been changed, that earlier naturalists classed the young /e^^as as a 

 crustacean, and the adult barnacle as a multivalve mollusk. And so, in- 

 deed, it is a mollusk, with a few crustacean characteristics not yet lost. 



The steps from this to the perfect mollusk are too plain to be dwelt 

 on here. Indeed, the difference is so small that, had not the larval cir- 

 rhiped been discovered, the position of the group would never have 

 been assailed. TJiey should, like human aspirants to rank, have con- 

 cealed their plebeian origin. If the observations of recent embryolo- 

 gists are to be credited, many mollusks and molluscoidea exhibit a sim- 

 ilar evolution. At any rate, even in undoubted mollusca, the elements 

 of their segmentarian origin are abundantly visible. 



