The only infallible conclusion is that the two groups are closely related and that they arose from a common ancestor. The walls of the tube are lined with mucus secreted by the mucous gland of the animal. Development mostly indirect through a free swimming tornaria larva. Most forms are drab coloured, though reddish tints are present, several species are luminescent due to mucus. Colour: Different species of this genus will show different colours. The buccal diverticulum or stomochord of hemichordates has been regarded as the equivalent of a notochord since the time of Bateson.
The nervous system in both the groups is intraepidermal. Filter feeders have mucous secreting glands and numerous cilia on their proboscis. The hemichordate tornaria larva appears like a modified trochosphere larva of polychaete worms. Below the nervous layer is a thick basement membrane made up of two lamellae pressed together. There is no post-anal tail in hemichordates. In some species the most anterior are overlapped by a posterior prolongation of the collar called the operculum.
It can be safely concluded that there is no representative of the notochord in hemichordates. Conclusion: The above affinities have led to the conclusion that echinoderms, hemichordates and chordates have arisen from a common ancestral stock, the dipleurula larva. They are, as mentioned above, closer to invertebrate phyla Echinodermata and Phoronida in their morphology and development than to the Chordates. In these genital ridges, gonads are situated. The proboscis is held out of the burrow entrance and organic particles are caught in the mucous which is swept to the mouth by the beating of the cilia. This structure, as predicted by Newman, has ceased to be regarded as notochord due to the following reasons: 1 The notochord is never formed as a diverticulum.
Etymology:- From the Greek Hemi for half and the Latin Chorda a chord. Their body consists of three parts; proboscis, collar and trunk. The number of eggs discharged at a time varies from few dozens to more than a thousand. The anus often possesses sphincter muscles. It is soft-bodied, slender marine burrowing animal. Excretion is mainly accomplished by a structure peculiar to the Enteropneusta called the glomerulus, a vascular complex placed on either side of the anterior portion of the stomochord, projecting into the proboscis-coelom. The archenteron opens to the outside through a blastopore.
The tentacles secrete mucous which is driven, along with the food particles trapped in it, to the mouth by the beating of the cilia. Its body wall is muscular. The ring vessel arises from the ventral vessel, located in the collar-trunk septum and it is connected with the dorsal vessel. The gastrula elongates along the anteroposterior axis. Trunk Coelom: The trunk coelom or metacoel has two closed cavities lying between the body wall and alimentary canal. Nervous System and Sense Organs 11. The epidermis of hemichordate is ciliated whereas it is nonciliated in chordates.
It is at this condition the egg is fertilizable. The buccal diverticulum is generally made of ordinary epithelial cells, while the notochord of vertebrates consists of large vacuolated cells. Class Thaliacae: They are planktonic. The dorsal cord projects into the collar coelom as the collar cord or neuro-cord- Nerve fibres run out from the ring. Nervous system primitive comprising mainly of an intra-epidermal nerve plexus. It tests the quality of food and the water entering the mouth.
In higher chordates, the pharyngeal region is very short with generally 5 or 6 pairs of gill-slits. Blood from the central sinus enters the glomerular sinuses of the proboscis complex. In both the coelom is of enterocoelous origin and it divides into three antero-posterior parts, which in hemichordates are called proboscis coelom protocoel , collar coelom mesocoel , and trunk coelom metacoel , while in echinoderms the three parts of the coelom are axocoel, hydrocoel, and somatocoel. The males also release their sperm into the water, the sperm swim and seek out the eggs which it is believed they detect chemically. Notochord: buccal diverticulum; stomochord: A diverticulum from the dorsal wall of the alimentary canal just behind the mouth, runs forward into the basal part of the proboscis, after sending out a small branch.
The two cavities are separated by an incomplete dorsal and a complete ventral mesentery. They are bilaterally symmetrical animals. There are certain resemblances between the nervous system of hemichordates and chordates, such as its position, formation of the dorsal nerve cord from the dorsal epidermis, and the hollow collar cord which often has a neuropore and is comparable with the neural cord of vertebrates. Nervous system is normally diffused. Hence Hyman has placed them in the invertebrates as an independent phylum. The blastopore marks the posterior end of the embryo.