The below mentioned article provides biology notes on Phylum Hemichordata.
(Gk. hemi- half, chordate- notochord)
Due to some similarities with chordates, some workers considered Hemichordata as a subphylum under phylum chordata. But, on grounds of its general organization, recent scientists, like Vander Horst (1939), Dawyd off (1948), Marcus (1958) and Hyman (1959), have given it the status of a phylum under nonchordata. The name “Hemichordata” is, however retained for the group.
1. Habitat:
They mostly live in burrows and are exclusively marine.
2. Body Form:
Body is soft, worm-like and is divisible into proboscis, collar and trunk.
3. Body wall:
It is with a single layered epidermis.
4. Coelom:
The hemichordates are enterocoelous.
5. Stomochord:
It is a hollow outgrowth, arises from the roof of the buccal cavity, called “buccal diverticulum”. It is present in the proboscis.
6. Digestive Tract:
It is complete.
7. Gill Slits:
These are narrow openings in the pharynx. When gill slits are present, they are one to several pairs. Gill slits are dorsal in position. They are lateral in chordates.
8. Respiration:
Respiration takes place through branchial portion of pharynx bearing gill slits.
9. Blood Vascular System:
It is “open type”, usually with a contractile heart vesicle and two longitudinal vessels, one dorsal and one ventral, inter connected by lateral vessels and sinuses. Blood is colourless.
10. Excretory organ:
It is a single proboscis gland or glomerulus situated in the proboscis.
11. Nervous system:
It is primitive, consisting mainly of an intra-epidermal nerve plexus. Balanaoglossus has both dorsal nerve cord and ventral nerve cord.
12. Sense organs:
Sensory cells of the epidermis act as sense organs.
13. Reproduction:
It is mostly sexual. Sexes are separate or united. Gonads are one to several pairs. Fertilization is external.
14. Development:
Except in some forms development is mostly indirect through a free swimming tornaria larva. Examples: Balanoglossus, Saccoglossus, Cephalodiscus and Rhabdopleura.
Balanoglossus— The Ancorn or Tongue Worm:
It is a marine animal which lives in U-shaped burrows in the bottom sand. It feeds on organic matter ingested along with sand. It is unisexual. Fertilization is external. Life cycle includes a tornaria larva. Power of regeneration is well marked.
It is worm like animal.
The body is divisible into three regions:
(i) Proboscis:
It appears as a tongue-like projection, hence the name “tongue worm”,
(ii) Collar:
It is the muscular region which bears mouth ventrally below the proboscis stalk,
(iii) Trunk:
It is divisible into three regions:
(a) The anerior branchiogenital region which has gill pores and gonads. The lateral regions containing the gonads are thin and flat and form genital wings.
(b) The middle hepatic region is marked externally with irregular elevations due to sacculations formed by projecting hepatic caeca of the intestine;
(c) The posterior abdominal region gradually tapers posteriorly and bears a terminal anus.
Asexual Reproduction in Animals:
1. Binary fission:
Binary fission is the division of the parent animal into two nearly equal sized daughter individuals. It occurs in Planaria.
2. Budding:
An outgrowth called bud appears due to multiplication of cells in the ectodermal layer. The bud grows in size. It is pinched off from the parent organism and establishes itself. Budding is common in Hydra and sponges.
3. Fragmentation:
The body of the parent may break into some pieces. Each piece develops into a whole animal. It is called fragmentation. It occurs in Planaria and Sponges.
4. Gemmule Formation:
In the fresh water sponge (e.g., Spongilla) and a few marine sponge buds are formed within the parent’s body. They are called gemmules (= internal buds). Gemmules come out of the sponge. The cells (archaeocytes) of gemmule form a sponge.
5. Strobilation:
During development, the gastrula changes to the planula (larva) in Aurelia (a jelly fish). The planula forms another larva, the scyphistoma. In winter and spring, the scyphistoma undergoes a process of transverse fission to form a number of ephyrae (larvae) so that the scyphistoma assumes the appearance of a pile of saucers.
The division of the scyphistoma larva of Aurelia into a number of ephyrae larvae by a series of transverse fissions is called strobilation. The ephyrae are detached and feed and grow. In due course of time the ephyrae metamorphose into jelly-fishes. Strobilation also occurs in the neck of Taenia.