The Mollusk Aborigines 



( Class Monoplacophora ) 



Until 1957 no mollusk had been found giving 

 more than token support to the scientists' hunch that, 

 in the remote past, a limpet and a clamworm had 

 shared a common ancestor. All known mollusks had 

 gone ahead with their evolution in ways that placed 

 no premium on a segmented body, whereas the anne- 

 lids had found special advantages in partitions isolat- 

 ing a series of chambers — each a part of the body 

 cavity. 



Then, among a collection of bottom animals 

 brought aboard the Danish research ship Galathea 

 in 1956, from nearly twelve thousand feet below the 

 surface of the Pacific Ocean some three hundred 

 miles from the nearest shore (Nicaragua), biologists 

 found some "living fossils" — ten complete, preserved 

 specimens and three empty shells of a kind of crea- 

 ture no one had ever seen before. They named it 

 Neopiliiui galatheae, and recognized it from its shell 

 as representing a type of life believed extinct since 

 the Devonian period of geological time, four hundred 

 million years ago. No animal discovered in recent 

 years has meant so much in scientific understanding. 



The deeps of the sea must hide many such treas- 

 ures. In December of 1958, four more specimens of 

 Neopilina were hauled up to daylight from more 

 than nineteen thousand feet below the surface. Until 

 the dredge of the American research vessel Veina ar- 

 rived, these mollusks had been living on the bottom 

 of an ocean valley known as the Peru-Chile Trench, 

 about one hundred miles from the coast of northern 

 Peru. This was more than thirteen hundred miles 

 from and seven thousand feet deeper than the earlier 

 find. The species collected aboard the Venio received 

 the name A', ewingi, to honor Dr. Maurice Ewing of 

 Columbia University, who had arranged the expedi- 

 tion as part of a long-term enthusiasm for deep-sea 

 biological exploration. 



Both kinds of Neopilina could easily be mistaken 

 for some kind of limpet with a small flat foot. The 

 largest of the thin, almost circular shells is about % 

 of an inch long and about •'•'] ,; of an inch high, like a 

 short stocking cap with the extra material drawn to a 

 low point in the middle at the front. 



Shells of this general style are known from the 

 early Paleozoic sedimentary rocks, and the only 

 strange thing about them is the pairs of little scars on 

 the inner surface, showing where muscles held the 

 animal to its armor. That these scars were (and are) 

 paired attracted no special attention until a pre- 

 served Neopilina became available for study. Then 

 the creature was seen to have a pair of gills on each 

 side of the foot under the mantle to correspond to 

 each pair of muscle attachments. And between the 



gills, paired excretory tubules open — tubules (ne- 

 phridia) far more like those of marine annelid worms 

 than those of any mollusk known. 



Neopilina cannot be said to be segmented, for its 

 body contains no crosswise partitions — but, in this 

 sense, neither can an adult leech. And no one has yet 

 seen the young of these mollusk aborigines. The du- 

 plication of parts, whether gills or excretory tubules 

 (nephridia) or shell-holding muscle bands, all show 

 a similarity to annelid worms. At the same time, a 

 pair of fleshy flaps on each side of the mouth suggest 

 the food-manipulating organs of a clam. A series of 

 short tentacles just posterior to the mouth, in front of 

 the foot, could well represent on a diminutive scale 

 the "arms" of an octopus or squid. 



Whether Neopilina creeps over the oozy bottom on 

 a thin cushion of secreted mucus or lies on its back 

 and uses the mucus film as a trap for food may even- 

 tually be learned. Its one-piece shell provides the 

 basis for the name devised in 1957 for the new class 

 Monoplacophora ("one-plate-bearer"). No doubt 

 other "living fossils" will come to light as explorations 

 continue in the dark depths of the sea. 



The Sea Cradles 



( Class A mp/iineiira ) 



If an animal clinging to a rock at the seashore 

 wears a shell consisting of eight transverse limy 

 plates, it is a sea cradle (chiton). The natives of the 

 West Indies call these exclusively marine animals 

 "sea beef," and sometimes collect them to cook as 

 food. American Indians on the Pacific coast used 

 them in this way too, and had available the largest sea 

 cradle in the world — as much as 13 inches long. 



The broadly oval foot of a sea cradle provides the 

 animal with a suction cup for clinging to rocks and a 

 means for slowly moving from place to place while 

 rasping algae from the rock face by repeated strokes 

 of the filelike radula. The edge of a sea cradle's man- 

 tle comes down around the foot, and wears an en- 

 circling girdle of minute limy plates suggesting a 

 coat of mail. 



Above the girdle, the mantle secretes the eight 

 valves of the shell proper, each fitted to its neighbor 

 in such a way that the animal can curl up into a ball 

 if detached from its support. While partly curled, the 

 inverted chiton may rock gently like a cradle. 



Sometimes the hard, whitened valves from a dead 

 sea cradle wash ashore separately and are called "sea 

 butterflies" because of the limy wings that provide 

 the hinge action between one plate and the next. In 

 some localities the chiton itself is known as the "but- 

 terfly fish." The series of shell valves might be as- 

 sumed to indicate segmentation. But it does not cor- 

 respond to the arrangement of bushy gills (six to 



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