Hemichordata
Due to some similarities with chordates, some workers (Bateson) considered this group as a subphylum of the phylum Chordata, representing its lowest group. But, on grounds of its general organization, some recent scientists, like Van der Horst, Dawydoff, Marcus, and Hyman, have given it the status of an independent invertebrate phylum. The name “Hemichordata” is, however, retained for the group. Tornaria larva of Balanoglossus was first described by J. Muller.
Distinguishing Features of Hemichordata
- They are mostly tubiculous living in burrows and are exclusively marine.
- The body is worm-like and is divided into proboscis, collar, and trunk.
- The body wall is with a single-layered epidermis.
- The hemichordates are enterococcus.
- Stomochord is a hollow protuberance, that arises from the roof of the buccal cavity, called the ‘buccal diverticulum’. It is present in the proboscis.
- The digestive Tract is complete.
- They are one to several pairs when gill slits are present. Gill slits are dorsal in position. They are lateral in chordates. Respiration takes place through the gills.
- The blood Vascular System is an open type, usually with a contractile heart vesicle and two longitudinal vessels, one dorsal and one ventral, interconnected by lateral vessels and sinuses. Blood is colorless.
- The excretory organ is a single glomerulus situated in the proboscis, hence called the proboscis gland.
- The earliest nervous system consists mainly of an intra-epidermal nerve plexus.
- Sensory cells of the epidermis act as sense organs.
- Sexes are separate. Reproduction is mostly sexual. Gonads are one too many pairs. Fertilization is external.
- Except in some forms, development is mostly indirect through a free-swimming tornaria larva.
Phylum Hemichordata
There are distinct animals with distinct structures and forms. Till now, a million species of animals have been introduced, and the urge for classification becomes all the more essential. The classification also assists in allowing a systematic position for newly introduced species. Animals have definite shapes and sizes. They are unbranched except for the sponges. Animal organs are generally internal, e.g., liver, heart, kidney, lungs, brain, stomach, etc.
The growth of an animal’s body is limited, and it stops long before death. The growth regions are not localized. They can move bodily from one place to another. They have sense organs, nervous systems, and excretory organs. There are different Phyla such as Porifera, Cnidaria, Ctenophora, Platyhelminthes, Nematode, Annelida, Arthropoda, Mollusca, Echinodermata, hemichordate, and Chordata. Let us have an overview of Phylum Hemichordata.