 |
| Phylum hemichordata : |
Once grouped as chordates, the Hemichordate are now a distinct phylum.
They are group of worm like marine animals with apparent and intriguing relationship with the echinoderms and the chordates.
The acorn worms (hemichordates) are believed to be a link between the vertebrate and invertebrates.
The hemichordata lacks the notochord which excludes them from the chordates,however,they exhibit the presence of the pharyngeal gill slits and in some species,a dorsal hollow nerve cord.
Classification of the phylum hemichordata
The phylum comprises of two classes :
*class enteropneusta
*class pterobranchia
Class enteropneusta : This is a cylindrical, soft bodied and worm like animal with ciliated body surfaces. They are acorn worms. They are common inhabitants of shallow water, with most species forming mucus lined burrows in Sandy or muddy sediments. The body is long and narrow and is divided into three distinct regions.
The anterior called the proboscis is followed by a narrow collar regions and then the bulk of the body called the trurik.
The three body regions correspond with typical deuterostome body division:protosome,mesosome and metasome e. g. Balanoglossus, Ptychodera.
 |
| Acorn worm(saccoglossu) |
Class pterobranchia : They exhibit a dorsal extension of the pharynx,forming an anterior stimichord just like the enteropneusta, which serves to support the surrounding tissues. However, they differ from the enteropneusts by possessing ciliated, anterior tentacles and a U shaped gut. Although, most members of this group have rigid tubes that they secretes e. g. Cephalodiscus.
No comments:
Post a Comment