- BIONOMTC CHARACTERS OF MOLLUSCA 1021 



which Hve in vast numbers in the pelagic district, usually some 

 distance from shore. While able to swim about in the water, they 

 are, nevertheless, at the mercy of the waves and currents. Their 

 food consists of pelagic organisms, and not uncommonly one species 

 of pteropod will prey upon another. They shun the light, descend- 

 ing during the day to the regions of perpetual twilight or even 

 darkness, some descending as far as 700 fathoms. Nearly all the 

 shelled pteropods of the present time -are confined to warmer 

 waters, and are especially abundant in the warm ocean currents. 

 Their shells often accumulate in vast numbers on the ocean bottom. 

 A veliger larva, similar to that of the gastropods, occurs. 



Cephalopoda. The cephalopods are marine nektonic or ben- 

 thonic molluscs inhabiting water of moderate depths. Swimming is 

 accomplished by the forcible ejection of water from the hyponome, 

 and probably also by the use of the arms. Among the dibranchiata 

 the majority of Sepioidea (Squids, Calamaries) are active swim- 

 mers, usually inhabiting the open sea, but appearing periodically on 

 the coasts in great shoals. They live mostly on small fish. The 

 Octopoda are less adapted to active swimming, lying usually in 

 wait for their prey on the sea-bottom or in crevices and hollows. 

 The Argonauta is, however, a partial exception to this, for, though 

 it crawls about on the sea-bottom like other octopods, it is often 

 met with swimming at or near the surface, by the ejection of the 

 water from its hyponome. Argonauta is, therefore, like other 

 cephalopods, at times a vagrant benthos, at others a nekton, in- 

 clining perhaps more to the latter, as do the decapods ; while 

 other octopods are commonly benthonic. Among the less active 

 decapods. Sepia may be mentioned as more normally a vagrant 

 benthonic form, crawling about on the sea-bottom, though able 

 to swim as well. A sedentary benthonic cephalopod is also known. 

 This is Spirula, which attaches itself to rocks like an actinia 

 (Agassiz, Walther), or lies partly buried in the mud, with its 

 beautiful coiled and chambered shell wholly concealed by the 

 fleshy parts. A perfect specimen was dredged oif Grenada in the 

 Caribbean by the Blake, from a depth of 950 fathoms. (Agassiz-i, 

 n:6i.) 



Spirula would seem to be a widely distributed form, judging 

 from the occurrence of its shell in almost all parts of the tropical 

 and temperate seas. The animal is very rare, however ; only one 

 specimen with soft tissues preserved having been obtained by the 

 Challenger expedition. This was taken close to the island of 

 Banda in 360 fathoms (Challenger Narrative). Altogether perhaps 

 only half a dozen animals with the soft parts preserved have been 



