Thepulpymatteroftheinternalcoatingsuddenlygroupeditselfintolines，someofwhichassumedaformradiatingfromacommoncentre；itthencontinued，withanirregularandrapidmovement，tocontractitself，sothatinthecourseofasecondthewholewasunitedintoaperfectlittlesphere，whichoccupiedthepositionoftheseptumatoneendofthenowquitehollowcase。Theformationofthegranularspherewashastenedbyanyaccidentalinjury。Imayadd，thatfrequentlyapairofthesebodieswereattachedtoeachother，asrepresentedabove，conebesidecone，atthatendwheretheseptumoccurs。
Iwilladdhereafewotherobservationsconnectedwiththediscolorationoftheseafromorganiccauses。OnthecoastofChile，afewleaguesnorthofConcepcion，theBeagleonedaypassedthroughgreatbandsofmuddywater，exactlylikethatofaswollenriver；andagain，adegreesouthofValparaiso，whenfiftymilesfromtheland，thesameappearancewasstillmoreextensive。
Someofthewaterplacedinaglasswasofapalereddishtint；and，examinedunderamicroscope，wasseentoswarmwithminuteanimalculadartingabout，andoftenexploding。Theirshapeisoval，andcontractedinthemiddlebyaringofvibratingcurvedciliae。Itwas，however，verydifficulttoexaminethemwithcare，foralmosttheinstantmotionceased，evenwhilecrossingthefieldofvision，theirbodiesburst。Sometimesbothendsburstatonce，sometimesonlyone，andaquantityofcoarse，brownish，granularmatterwasejected。Theanimalaninstantbeforeburstingexpandedtohalfagainitsnaturalsize；andtheexplosiontookplaceaboutfifteensecondsaftertherapidprogressivemotionhadceased：inafewcasesitwasprecededforashortintervalbyarotatorymovementonthelongeraxis。Abouttwominutesafteranynumberwereisolatedinadropofwater，theythusperished。
Theanimalsmovewiththenarrowapexforwards，bytheaidoftheirvibratoryciliae，andgenerallybyrapidstarts。Theyareexceedinglyminute，andquiteinvisibletothenakedeye，onlycoveringaspaceequaltothesquareofthethousandthofaninch。Theirnumberswereinfinite；forthesmallestdropofwaterwhichIcouldremovecontainedverymany。Inonedaywepassedthroughtwospacesofwaterthusstained，oneofwhichalonemusthaveextendedoverseveralsquaremiles。Whatincalculablenumbersofthesemicroscopicalanimals！Thecolourofthewater，asseenatsomedistance，waslikethatofariverwhichhasflowedthrougharedclaydistrict，butundertheshadeofthevessel’ssideitwasquiteasdarkaschocolate。
Thelinewheretheredandbluewaterjoinedwasdistinctlydefined。Theweatherforsomedayspreviouslyhadbeencalm，andtheoceanabounded，toanunusualdegree，withlivingcreatures[note9]。