Page:Popular Science Monthly Volume 5.djvu/590

572 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 mollusk 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 larval 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! she 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 superior 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 entomostracous its nearest relations. Take one generally known, the genus lepas—the common goose-barnacle—in extreme youth, an exceedingly active little shrimp-like tenant of the deep. During the latter part of this stage, the hypertrophied segments of the thorax continuing 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 exactly at right angles to the original axis of vitality in the young crustacean. 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 gemmations 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 lepas as a crustacean, and the adult barnacle as a multivalve mollusk. And so, indeed, 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 cirrhiped been discovered, the position of the group would never have been assailed. They should, like human aspirants to rank, have concealed their plebeian origin. If the observations of recent embryologists are to be credited, many mollusks and molluscoidea exhibit a similar evolution. At any rate, even in undoubted mollusca, the elements of their segmentarian origin are abundantly visible.