The teredo writing an essay help first undergoes segmentation within the egg, and then appears as an active free-swimming best writing paper veliger, C, differing from the young cockle only in that there no lash-like flagellum. Then its mantle and shell are formed, and when five and a dissertation outline write my thesis statement for me half days old the shells have well-nigh invested the whole body. Next the foot the ship-worm developed, and the velum becomes a crown cilia. Then, as the young animal seeks the wooden pile wherein destined bore and ensconce itself, the shells come into play as excavating organs, and, with the growth the elongated body, ship-worm development may said conclude. Thus find that the course bivalve development distinctly enough mba essay services marked. Only in one or two cases such as that the freshwater mussels, Unio the veliger-stage suppressed. But this latter fact will cause no surprise the student development, who well aware that the effects varying conditions the developing many changes in an early life-history, and in rendering obscure many phases in the panorama individual Coming next the gasteropods, which the limpets, whelks, snails, slugs, and the univalve shell-fish at large, are examples, find a striking buy and sell research papers similarity in their early history the development just sketched.
A mussel OT oyster or other bivalve has, as every one knows, no distinct head. This may the result degradation. But in the snails, whelks, and their neighbours the head plainly enough marked, although in certain low forms the gasteropod class this head development may not at all prominent. Such lower members are illustrated the Dentalium or tooth shell, otherwise often named the elephant's tusk-shell, from its obvious resemblance the latter structure.
In the early history Dentalium, find obvious resemblances the development the bivalves.
First, segmentation or division the egg takes place A. Next, the young tooth-shell escaping from the egg appears as a rounded body, and possesses tufts cilia for swimming, and likewise has a fLigellum in front. The body then lengthens and develops seven circlets cilia C, the resemblance between the young tooth-shell in this guise and an embryo worm, being unmistakeable. Then the shell formed the mantle 1, D, a as before, english paraphrasing and the cilia form a velum at the upper extremity the body, the young condition the bivalve being closely imitated at this stage.
The shell, at first open below, unites its lower edges form the toothlike structure the perfect animal, and with the further growth the internal organs E Dentalium becomes the mature animal. There cannot exist a doubt that, as the lowest gasteropod, and as a poor relation the higher whelks and snails, Dentalium's life-history need a research paper done shows, as might expected, the closest approach, firstly, animals lower grade than mollusca, writing essays services and, secondly, at a more advanced stage that the veliger 1, C, D the bivalves themselves. latter forms are found adhering the rocks and stones at low water, like the neighbouring limpets. They agree with the limpets in being gastcropods but their structure if any thing, lower than that not univalve, but composed no fewer than eight pieces, A, arranged one after the other, the animal's back.
No definite head, however, found in the chitons, this lack front extremity being, as before, medical writing services a proof lowness and democracy in the scale gasteropod society. The general aspect a chiton unquestionably more like that an articulated or jointed animal than a mollusc, in which latter not expect see segments any kind represented. It likewise a fact much interest that these chitons are a remarkably ancient group the gasteropod class. They may, true, regarded, the strict rules comparative anatomy as lower organisms than the whelks and their relations. But if antiquity origin in the gasleropods, as esteemed in higher circles, a criterion respectabiHiy, then the chiton race may claim a superior rank nuany their neighbours, and may maintain that when the univalve race was but in the infancy its development, they possessed a stable and well-founded family connection.
© Gouvernement du Québec, 2016