GHAPTER LIX 



THE SEA BUTTERFLIES 



• Class Pteropoda 



A PELAGic'group of mollusks, reaching shore only by accident, 

 as when storm-driven. They live in communities, in all seas; 

 the Arctic species are the most highly coloured. They rise to 

 the surface at" twilight; rarely specimens come up indaytirne. 

 They feed upon microscopic mollusks and crustaceans. 



The pterbpbds are all small mollusks, naked or with small, 

 transparent shells, internal or external. . Some are trumpet- 

 shaped, some cylindrical, with needle-like shells. Others are 

 pyramidal or globular. Shells like those of the peai^ly nautilus, 

 the purples and the Hungarian cap occur among the spiral forms. 

 The foot is dilated into two :wing-like'swiniming disks, or these 

 disks occur as accessory organs of locomotion, the foot being 

 rudimentary. The/'position of the body, in swimming is "wrong 

 side up," the abdomen uppermost. The head has tentacles which 

 bear organs of hearing and smell, but not of sight. The large 

 proboscis has a Hngual ribbon armed with recurved spines. 

 There are sometimes grasping organs. Gills are internal or ex- 

 ternal. Young pteropods swim by a velum until the adult 

 swimming lobes appear. In all genera the young have shells. 



The interesting genus, Firola, has a few species, with slim 

 fusiform bodies, propelled by a ventral and a caudal fin. The 

 gill rises unprotected above the tail. These creatures exhibit 

 their entire structure without reserve. The circulation of the 

 blood is -traceable through the transparent tissues. Well devel- 

 oped hearing organs enable the creatures, to detect enemies. 

 Their flight is swift, and the loss of the head does not seem to 

 deter them at all. Adults have no shells. 



They, inhabit the Mediterranean Sea, and, the Atlantic and 

 Pacific oceans. , . . 



Spirialis Flemingii, seen ofi' Nahant in considerable abun- 

 dance in 1863, were- studied by Alexander Agassiz. He observed 

 that they came to the surface at high tide when it bccured directly 



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