The chitons begin their fossil history in the lower Silurian rocks, and appear at the present time with but little variation from their past structure. They are, therefore, unquestionably an ancient series beings, which have most probably sprung from a far back root-stock, whence the gasteropods themselves, and other molluscs hkewise, may have branched off become the superior shelled races and tribes to-day.
What, then, the course chiion development? As should expect, much more with the early history, coursework resources From the egg, the infant chiton 11, A issues forth as an oval speck possessing a circle cilia suirounding its body near the front extremity and hkewise bearing a tuft cilia its the best essay writing services head. The hkeness between the young chiton d the young cockle, A clearly traceable. An eye-spot soon appears each side the ciliated circlet, and the body next becomes annulated or ringed in appearance, such an aspect reminding one most forcibly the young stages the worms 1.
Even when the young chiton exists in this free swimming state, the segments the shell begin appear C, and correspond with the rings into which the lan-al body divided whilst subsequently the broad foot developed, and the animal settles down into a sedentary and placid existence the rocks and stones the coast Chiton development thus tells a tale early origin, and alliance with the worm research papers buy typical phases.
The eggs the pond snail are found in Jirne deposited the under surface the leaves water-plants, enclosed in capsules containing a white jelly-like matter.
The egg undergoes complete yolk segmentation, and then the gastrula stage, with its two layers 1, repeated in all animals from sponge man appears the mouth this sac closing as the young form passes enter the veliger stage, in will someone write my paper for me which the body oval, and possesses a ciliated ridge. This latter stage has also received the name trochosphere. Ultimately the foot developed, then the shell appears, and in due time the snail form assumed.
In the pond snail, as a high form mollusc, unquestionably find a veliger stage, reminding the similar phase in other and lower univalves and in bivalves.
It a noteworthy fact that the land snails and slugs not show the velum, notwithstanding their apparent nearness the pond snail. The suppression the veliger stage here does not surprise On the contrary, are fully prepared for such lapses and omissions in development the consideration, already enforced, that altered need help with term paper ways life must inevitably produce a changed life-history. Such omissions, in fact, exactly answer the expectation the evolutionist and their absence would indeed prove a veritable stumbling-block his hypothesis.
In the top shells Trochus familiar enough as native species, may mentioned that the veliger stage 1, A, or that the trochosphere, well represented, whilst in this stage also marvellously like the embryos certain worms, and also resembles that animalcules. Later the velum becomes more prominent, and as the shell develops, the larva assumes the likeness the young top-shelL Such a life-history worth recording, even in a cursory fashion, if only emphasise the fact that, even in some imdoubted univalves, the likeness lower worms remarkable.
custom essays essay help Certain other univalves somewhat different structure from those whose development has just been described, may now noticed. These latter are the so-called naked gasteropods, in which a shell either rudimentary or wanting altogether. But the curious fact remains that, whether a shell present or not, these animals invariably possess that structure in their embryonic state. This shell, which thus never destined developed, an illustration rudimentary organs, which, like the teeth the unborn whalebone whale dissertation writers possessing no teeth whatever in its adult state have a reference a past stale things. These teeth and the rudimentary shell are heritages derived from ancestors which had well developed teeth and shells respectively. Otherwise, and any other theory nature, their mere existence a hopeless and insoluble puzzle. The shell-less univalves which refer are often familiarly named sea slugs, sea lemons, and the like. By naturalists they are placed in such genera as Doris, olis T 1, Apfysia, Other examples these molluscs are included in the genus Bulia, or that the Bubble shells, possessing a delicate shell, and Aplysia, or that the Sea-hares, famed old as an ingredient in classical poison-cups. Bulla and the Sea-hares possess each a thin shell, which, however, a secondary growth, and does not represent the true shell or that developed in early life.
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