60i 



Pteropoda. — The pteropo.Is are marine ])lanktonic mol- 

 luscs, which Hve in vast numbers in the pehin-ic district, 

 usually at some distance from shore. While al)le to swim 

 about in the water, they are nevertheless at the mercy 

 of waves and currents. Their food consists of ])elagic 

 organisms, and not nncommonlj' one species of pteropod 

 will prey upon another. They shun the light, descending 

 during the day to the regions of perpetual twilight or even 

 darkness, some descending as low as 700 fathoms. Nearly 

 all the shelled pteropods of the present time are confined to 

 the warmer waters, and they are especially abundant in the 

 warm ocean currents. A veliger larva similar to that of 

 gastropods occurs. 



Ct^phnlopochi. — The cephalopods are marine nektonic or 

 benthonic molluscs, inhabiting water of moderate depths. 

 Swimming is accomplished by the forcible ejection of w^ater 

 fi-om the hyponome, and probably also by the use of the arms. 

 Among the Dibranchiata the majority of Decapoda (Squids. 

 Calamaries)are active swimmers, usually inhabiting theo})en 

 sea, but appearing periodically on the coasts in great shoals. 

 They live mostly on small fish. The Octo|)oda are less 

 adapted to active swimming, lying usually in wait for their 

 prey on the sea bottom or in crevices and holh^ws. The 

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

 though it crawls about on the sea bottom, like other octo- 

 pods, it is often met with swimming near or at 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 — inclining perhaps more to the 

 latter, as do most of the decapods ; while other octopods are 

 more commonly benthonic. Among the less active deca- 

 pods. Sepia may be mentioned as more normally a vagrant 

 benthonic form, cra^^'ling 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 whollv 



