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VIII. THE PHYLLOSOMA OR GLASS CRAB LARVA OF SCYLLARUS. 



Pelagic animals arc, without exception, of the highest beauty, 

 but, saving for the wondrous loveliness of the pelagic Ccelenterates 

 the elfin-locked Medusids, the glassy Cydippe, and the phosphorescent 

 and elaborate Siphonophores, what can compare with the quaint 

 " Glass Crabs " of the older writers. These, long known by the name 

 of Phyllosoma, were considered a separate group of animals, till 

 Couch in 1857 hinted vaguely to the British Association that they 

 were in reality the larval forms of certain Decapod Crustaceans, 

 notably of Pcdinurus vulgaris, the Common Crawfish. Couch had 

 been studying for some years the development of the larger Crus- 

 taceans, and the pity is, that through inadequate training he was 

 unable to utilize to full, or even moderate advantage, the oppor- 

 tunities that the practically unexploited field of this study then 

 afforded. 



Couch's figures were anything but accurate, and his work did 

 little but point the way. Indeed not till so long after as 1870, was 

 the full value of the relationship definitely traced and worked out, 

 and the achievement is one of the many that Biology owes to the 

 indefatigable perseverance of that prince among naturalists the 

 veteran Dr. Dohrn. 



In that year Dr. Dohrn detailed the development of a very 

 near relative of Palinurus, viz. of Scyllarus arctus, while in the egg, 

 and also during a short period in its free swimming life, showing 

 that there is absolute identity between the hatched larva and a 

 certain form of Phyllosome. 



Other naturalists have extended these observations, and to-day 

 the only points requiring much elucidation, are the final metamor- 

 phoses just prior to the period when the Phyllosome larva changes to 

 the adult. A limit seems to be set to the age to which the larvaa 

 can be reared in confinement. For a while all goes well, then the 

 captives begin to find some necessary condition of life to be wanting, 

 and rapidly decrease in numbers, till not one survives. 



But before describing the larval form, let us glance at the chief 

 points in the anatomy of the adult. Scyllarus arctus is one of the 

 rarest of the large Crustaceans living in British waters, and like the 

 Squillidse, found only in the English Channel ; just on the northern 

 boundary of the great Mediterranean zone just where this zone 

 grades into that of the northern regions. Its captures on the 

 English coast can be traced in paragraphs in scientific journals, and 

 only in the Channel Islands is it frequently enough met with to 

 receive a vernacular cognomen. In these islands the fishermen speak 

 of it indifferently as the Bastard Crawfish and the Square-nosed 



