The larva grows from the anal end as an elongated cylinder which forms segments of the body by metameric segmentation.The anterior end of the body possesses a distinct, prominent and well developed head which consists of two main parts, prostomium and peristomium.In the middle of each bundle of setae and deeply embedded in the parapodium is found stout, straight, thick and dark coloured chitinous rod the aciculum which projects only a short distance but does not project beyond the outer edge or the parapodium. Fertilizing Capacity Of Sperm From Arenicola Marina A Nereis . There are nerves between adjacent segments, and excitation of a muscle layer in one segment leads to excitation of the same layer in other segments.Each septum has a double layer of coelomic epithelium containing muscles and connective tissue. Because of such changes the posterior modified segments of the body represent sexual region or epitoke and anterior segments without any change represent non-sexual region or atoke. The masticated food is pushed onwards inside the gut by rhythmic waves of contraction passing over the wall of alimentary canal from anterior to posterior end.These are long slender structures present on the anterior side of the peristomium. The two acicula constitute the endoskeleton of parapodium and serve to support and for attachment of the setal muscles. Thus, the sexually mature Neanthes having these two regions in the body is called Heteroneanthes (= Heteronereis).4. The middle region of the brain has two small lobes called corpora pedunculata which are association centres and coordinate all the impulses coming to brain.This pattern of division is called spiral cleavage in which any one cell lies between two blastomeres or below it, and at first the cleavage planes are oblique to the polar axis (axis between animal and vegetal poles).
Outer surface of epidermis is coated with a layer of tough cuticle which carries a number of small pores for glands. The peristomium is formed by the fusion of first two body segments, it forms the lateral and ventral margins of the mouth.1. The head consists of two parts: a roughly triangular anterior lobe—the prostomium—and a posterior ring-like portion—the peristomium. The first two parts of parapodia have no notopodial setae. These are tactile organs.
Neanthes Virens shows generally two types of locomotion, viz., slow creeping and fast creeping but sometimes it swims in water.3. The epidermis is richly vascularized and helps in respiration.Gills or any other special organs of respiration are lacking in Neanthes. Since the parapodia are highly vascularized, hence, they also serve the function of respiration.In Neanthes Virens, the excretion is carried out by means of special type of coiled tube-like structures, the nephridia, one pair of which is found in each segment except the first and the last segment.The proboscis may be everted at times only partly, revealing only the buccal cavity, this happens when the worm is digging or feeding on the surface mud, this is brought about by the pressure of the coelomic fluid.Between the ectoderm and gut is a large cavity, the blastocoel having mesencyme cells, larval mesoderm and a pair of larval nephridia, each made of two hollow cells, one of which contains a flame of cilia, there is an otocyst near the nephridia.Each parapodium is a biramous structure consisting of two parts-an upper or dorsal blade, the notopodium and a lower or ventral blade, the neuropodium. But each Para podium has other muscles also, the largest parapodial muscles arise from the circular muscles of the body wall and are joined to the acicula, they extend the acicula and Para podium. This type of mode of feeding is known as raptorial feeding.The succeeding cleavages are unequal and at right angles to the first two divisions, they form three quartets of micromeres which are divided off from the fourth quartet of macromeres, the micromeres lie towards the animal pole and the macromeres, at the vegetal pole.These changes are induced by hormones secreted into a plexus of blood vessels lying below the brain. The cells eventually form a gastrula, then the three germinal layers are formed.The brain has numerous nerve cells or neurons and nerve fibres and is large in size due to an active life.
Nereis spp.
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