BSc 2nd Year Zoology Hemichordata Notes Study Material
BSc 2nd Year Zoology Hemichordata Notes Study Material: BSc is a three-year program in most universities. Some of the universities also offer BSc Honours. After getting enrolled for BSc, there are certain things you require the most to get better grades/marks in BSc. Out of those, there are BSc Study Material, BSc Sample Model Practice Mock Question Answer Papers along with BSc Previous Year Papers. At gurujistudy.com you can easily get all these study materials and notes for free. Here in this post, we are happy to provide you with BSc 2nd Year Zoology Hemichordata Notes Study Material.
BSc 2nd Year Zoology Hemichordata Notes Study Material
General Characters
- Exclusively marine, solitary, or colonial, mostly tubicolous.
- Body soft, fragile, vermiform, unsegmented, bilaterally symmetrical, and triploblastic.
- The body is typically divided into 3 distinct regions; proboscis, collar, and trunk.
- The body wall of a single layered, epidermis with mucous glands. No dermis.
- Coelom enterococcus is usually divided into protocoel, mesocoel, and metacoel, which corresponds to 3 body regions.
- Digestive tube complete, straight or U-shaped.
- Foregut gives out a hollow buccal diverticulum into proboscis, earlier considered as ‘notochord’.
- Dorso-lateral pharyngeal gill slits, when present, one to several pairs. Ciliary filter feeders.
- The circulatory system is simple and open, including a dorsal heart and two longitudinal vessels, one dorsal and one ventral.
- Excretion by a single probosci’s gland or glomerulus connected to blood vessels.
- The nervous system is primitive and consists mainly of a subepidermal nerve plexus. Dorsal collar nerve cord hollow.
- Reproduction is mainly sexual. Sexes are usually separate. Gonads one to several pairs.
- Fertilization external, in seawater. Development direct or indirect with a free-swimming tornaria larva.
Classification
Hemichordata includes about 80 known species which are generally grouped under two classes, Enteropneusta and Pterobranchia. Besides, two more classes are included by some, as below.
Class 1. Enteropneusta
(Gr. enteron, gut + pneustos, breathed)
- Solitary, free-swimming, or burrowing animals, commonly called the ‘acorn’ or ‘tongue worms’.(Zoology Hemichordata Notes Study Material)
- Body elongated, vermiform, with no stalk.
- Proboscis cylindrical and tapering.
- Collar without ciliated arms (lophophore).
- The alimentary canal is straight. Mouth and anus at opposite ends. Filter feeding.
- Several pairs of U-shaped gill slits.
- Sexes separate. Gonads are numerous, and sac-like.
- The development includes tornaria larva in some. Asexual reproduction lacking.
Examples: Balanoglossus, Saccoglossus (= Dolichoglossus), Protoglossus, Prychodera, Spengelia.
Class 2. Pterobranchia
(Gr. pteron, feather + branchion, gill)
- Solitary or colonial, sessile, and tubicolous animals living inside secreted chitinous tubes.
- Body short, compact, with stalk for attachment.
- Proboscis shield-like.
- Collar-bearing ciliated arms (lophophore).
- Alimentary canal U-shaped. Anus dorsal lying near the mouth. Ciliary feeding.
- Gill slits one pair or absent, never U-shaped.
- Sexes separate or united. Gonads 1 or 1 pair.
- Development director with a larval stage. Asexual reproduction by budding in some.
Order 1. Rhabdopleurida
- Colonial, zooids are connected by a stolon.
- Collar with two tentaculated arms.
- Gill-slits absent.
- Gonad single.
Example: Single genus Rhabdopleura.
Order 2. Cephalodiscida
- Solitary or several zooids living unconnected in a common gelatinous case.
- Collar with several tentaculated arms.
- Gill slits single pair.
- Gonads single pair.
Examples: Cephalodiscus, Aruburia.
Class 3. Planctosphaeroidea
This class is represented by a few small, rounded, transparent, and pelagic larvae, supposed to be specialized tornaria of some unknown hemichordate termed Planctosphaera pelagic. The larval body is covered by extensively branched ciliary bands and its alimentary canal is L-shaped. (BSc 2nd Year Zoology Hemichordata Notes Study Material)
Class 4. Graptolite
The fossil graptolites (e.g. Dendrograptus) were abundant in the Ordovician and Silurian periods and often placed as an extinct class under Hemichordata. Their tubular chitinous skeleton and colonial habits show an affinity with Rhabdopleura.
Other Hemichordates
- Saccoglossus: It is a typical enteropneust genus very much similar to Balanoglossus in habitat, habits, and structure. It is a marine, slender, soft-bodied tubicolous tongue worm living in spirally twisted burrows. The body has the usual three divisions-proboscis, collar, and trunk. (BSc 2nd Year Zoology Hemichordata Notes Study Material)
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Saccoglossus And Ptychodera
The proboscis is exceptionally long and pointed than in other tongue worms. The posterior rim of the collar hangs like an operculum over the anterior end of the trunk covering the first 3 or 4 pairs of gill pores. Genital wings and hepatic caeca, so well-developed in Balanoglossus, are absent.
Mature gonads are yellow in males and grey in females and their position is marked externally by dorsolateral genital folds in the middle part of the trunk. Synapticula is not present so that tongue bars hang freely in their gill slits. (BSc 2nd Year Zoology Hemichordata Notes Study Material)
Development is direct without a free-swimming tornaria larva. It occurs almost universally. Saccoglossus pygmaeus, measuring 2 to 3 cm in length, represents the smallest known species of Enteropneusta.
- Ptychodera: This genus also bears a close resemblance to Balanoglossus, ecologically as well as morphologically and embryologically. Its proboscis and collar are somewhat shorter, but the trunk possesses conspicuous genital wings and hepatic sacculations. Development is indirect involving a free-swimming tornaria larva.
- Rhabdopleura: It is a marine, sedentary and colonial interbranch mainly found in the North Atlantic. The colony consists of horizontal branching gelatinous tubes, forming the coenoecium, which remain attached on hard substratum such as stones, corals, mollusc shells, sponges, etc. Erect tubes, about 6-7 mm in height, arise at short intervals, each housing an individual or zooid of the colony. The tubes are ringed, membranous and secreted by the zooids. A zooid is minute, hardly 1 mm long and occupies the distal part of the tube. Proboscis is disc-shaped. Collar bears a pair of hollow elongated arms beset with numerous fine ciliated tentacles for food collection.
The alimentary canal is U-shaped so that anus lies near the mouth. Gill clefts and glomerulus are absent. Sexes are separate but a colony has both male and female individuals. A single gonad is present on the right side of the trunk. Basally the trunk of each zooid is attached by a long contractile muscular stalk to a common cord of living substance, called black stolen or pectocaulus, running inside the horizontal tubes. The stalks can quickly contract spirally so as to withdraw the zooids into coenoecium for protection. The black stolon also forms new individuals asexually by budding. (BSc 2nd Year Zoology Hemichordata Notes Study Material)
- Cephalodiscus: It is a sedentary and gregarious pterobranch found mainly in the seas of Southern Hemisphere at depths of 50 to 650 m. Several zooids live in separate upright gelatinious tubes secreted by them and embedded in a common matrix called coenoecium, fixed permanently to substratum. (BSc 2nd Year Zoology Hemichordata Notes Study Material)
Foreign materials such as sand grains, sponge spicules, molluscan shells, etc. also adhere to the coenoecium. The zooids remain unconnected organically and thus do not constitute a true colony. Each zooid is 2 to 3 mm long and has the usual three body divisions-proboscis, collar and trunk. Proboscis is shield-shaped, overhanging the mouth. (BSc 2nd Year Zoology Hemichordata Notes Study Material)
Its cavity, the proboscis coelom, opens out through two proboscis pores. The collar bears 8-16 hollow arms (lophophore), which in female are beset with numerous fine, pinnately arranged and heavily ciliated tentacles used for food capture.
The tips of tentacles bear glandular knobs. The trunk is short and plump, bearing a single pair of gill-slits without skeletal support. Alimentary canal is U-shaped, with ventral mouth and dorsal anus located at the distal end. A narrow elongated contractile stalk arising from the aboral end attaches the zooid to its tube. (BSc 2nd Year Zoology Hemichordata Notes Study Material)
Sexes are separate, gonads single pair and development direct. Asexual reproduction also takes place by buds arising from stalk and soon becoming free. (BSc 2nd Year Zoology Hemichordata Notes Study Material)
- Atubaria: Sato first described Atubaria in 1936.
It is a sedentary and solitary pterobranch genus clinging to hydroid colonies by its long stalk and closely resembling with Cephatodiscus. A coenoecium is lacking. The zooid measures 1-5 mm in length with usual three divisions of the body.
The collar carries four pairs of tentaculated arms of which second pair distally has rod-like terminations devoid of tentacles. One pair of pharyngeal gill-slits are present. (BSc 2nd Year Zoology Hemichordata Notes Study Material)
BSc 2nd Year Zoology Hemichordata Notes Study Material
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