Now, in these naked gasteropods, there a well-marked veliger stage. Moreover, if the development such a form as glis, or its neighbours the Sea Lemon tribe, studied, the young form observed at one stage its career present a singular and highly characteristic appearance. It possesses a velum, consisting two well-marked lobes, A, richly ciliated, and means which swims rapidly through the fea, whilst the animal's foot and its shell are also readily observable. Far away in the Northern seas, the Arctic voyager may sometimes sail for days, or rather for nights, through water which may discoloured the innumerable myriads small organisms floating its surface. Each these beings veiy small size certainly under an inch in length at a maximum measurement and each paddles or flaps its way through the doctoral dissertation writing help sea means a pair wing-like fins attached the sides the neck.
Such are the Sea-butterflies, or Pteropoda and 1, already mentioned as a class the molkiscan group.
Their title regarded as shellfish rests research dissertation the cheap essays for sale fact that, besides agreeing with other molluscan characters, they may possess a delicate glassy shell,C but this structure may, at the same time, wanting, and a id may also indistinctly represented the latter fact indicating, as have seen, a position inferiority in the molluscan scale. Now, when a pteropod even cursorily regarded in the possession its wings or fins, borne the sides its neck, its resemblance the young, A some the naked gasteropods, how to write thesis proposal such Eolis, both essay writer cheap close and unmistakeable.
In their development the pteropods possess a velum, like most univalves.
This velum believed good authorities remain developed, and constitute the wings or fins these animals but other authorities their fins are believed represent certain side-lobes the molluscali body, and as such are regarded this second theory A with the adult pteropod, see the closest artinity and relationship between them.
The pteropod in this view represents a permanent larval or arrested gasteropod. Both have In some Byeioays Development.
arisen, if the story told development worthy credit, from a common root-stock, which the veliger-stage the transient representative.
Both have developed in parallel, may in corresponding and similar grooves.
But the gasteropod online custom writing services has been evolved beyond its veliger-stage assume a higher place in the animal series whilst the pteropod has been arrested in its development at this stage, and has assumed, with possibly a little fixation its characters, that larval condition as the badge and mark adult structure.
Passing, last all, a lower group molluscs that the Brachiopodtty or Lampshells iqa may find through these latter forms a passage the help me write a thesis sentence still lower and more primitive stock from which the moUuscan group may presumed have originated. The brachiopods form a scarce group shellfish in our present seas, but in past epochs this world's history they were abundantly represented. The Silurian rocks, mention but one group formations, literally teem with their fossil representatives, whilst the paucity these shells in existing waters matter zoological notoriety. These Lampshells are, therefore, an extremely ancient group living beings. That they are inferior in many phases structure the common bivalves such as our oysters and cockles matter fact. Hence the development these Lampshells may presumed, priori grounds, fraught with meaning and information as the descent and origin the mollusca at large. Let therefore, endeavour follow out the researches Morse the development these singularly interesting forms. Studying one species Terebratulina the common Lampshell the American coast the first free-swimming stage that an elongated body 1, which divides itself crosswise into three rings or segments C, D, the front one which becomes provided with long actively-moving cilia. Eye-spots essay help chat room also appear the front segment, and the likeness the young lampshell an embryo-worm at this stage plainly apparent Nor the likeness lessened, when the middle segment found develop four bundles setcBy or bristles, such as appear in the worms. Then succeeds the stage fixation.
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