Saturday, September 3, 2011

Salient features and affinities of Onychophora and Hemichordata


    • Affinities of Balanoglossus with Annelida & Echinodermata.

    Spengel (1893) suggested affinities of Hemichordata (Balanoglossus) with Annelida as follows:-
    1. Body vermiform & coelomate.
    2. Burrowing habit, tubicolous life & ingesting mud which is passed out as castings through anus.
    3. Collar of Balanoglossus is similar to clitellum of earthworm.
    4. Proboscis & prostomium is similar & preoral.
    1. The arrangement of blood vessel is similar.
    1. Dorsal position of heart.
    1. Tornaria larva of Balanoglossus shows several structural resemblances with the Trochophore larva of Annelida in being pelagic, ciliated, with apical plate, eye spots, sensory cilia & well developed alimentary canal with similar parts.
    Affinities of Balanoglossus with Echinodermata:-
    1. Adult resemblances:-
    Adult Balanoglossus & echinoderms are structurally quite different. So it is difficult to draw their phylogenetic relationship between themselves. However some resemblances are :-
    1. Enterocoelic origin of coelom.
    2. Heart vesicles & glomerulus of Balanoglossus are considered homologous to the dorsal sac & axial gland of echinoderm.
    1. Nervous system poorly developed & forms epidermal nerve plexus.
    1. Proteins & phosphagens present in Balanoglossus closely resemble those of echinoderms.
    1. Common habits, echological niches & remarkable power of regeneration.
    1. Larval resemblances:-
    Tornaria larva of Balanoglossus has striking, structural similarities with Bipinnaria larva of echinoderms.
    1. Small, pelagic, transparent & oval.
    2. Identical ciliated bands taking up a similar twisted course.
    3. Enterocoelic origin & similar development of coelom.
    4. Proboscis coelom of Tornaria is comparable to hydrocoel of echinoderm Dipleurula.
    1. Deuterostome & digestive tract is complete with mouth & anus & same parts.

    • Discuss & comment on echinoderm affinities of Balanoglossus.  Justify its inclusion under the phylum Chordata.

    • Echinoderm affinities of Balanoglossus;-
    1. Adult resemblances:-
    Adult Balanoglossus & echinoderms are structurally quite different. So it is difficult to draw their phylogenetic relationship between themselves. However some resemblances are :-
    1. Enterocoelic origin of coelom.
    1. Heart vesicles & glomerulus of Balanoglossus are considered homologous to the dorsal sac & axial gland of echinoderm.
    1. nervous system poorly developed & forms epidermal nerve plexus.
    1. Proteins & phosphagens present in Balanoglossus closely resemble those of echinoderms.
    1. Common habits, echological niches & remarkable power of regeneration.
    1. Larval resemblances:-
    Tornaria larva of Balanoglossus has striking, structural similarities with bipinnaria larva of echinoderms.
    1. Small, pelagic, transparent & oval.
    2. Identical ciliated bands taking up a similar twisted course.
    1. Enterocoelic origin & similar development of coelom.
    1. Proboscis coelom of Tornaria is comparable to hydrocoel of echinoderm Dipleurula.
    1. Deuterostome & digestive tract is complete with mouth & anus & same parts.
    Comment;-
    However the Tornaria larva shows presence of apical plate with sensory hairs , eyespots & telotroch which are absent in echinoderm larvae.
    The protocoel is single in Tornaria larva but paired in echinoderm larva. This raises the doubts about the echinoderm affinities of Balanoglossus.
    Fell (1963) & others believe that their larval similarities are only because of convergent evolution due to same mode of habits & habitats.

    Inclusion of Balanoglossus under phylum chordata is not universally accepted.
    Among 3 fundamental chordate characteristics the nature of notochord is really questionable. Recent workers do not accept the notochordal nature of buccal diverticulum.
    The nervous system, in general, is typically non chordate type excepting the presence of lumen in collar nerve cord.
    The only important chordate feature is the gill slits.
    However recent DNA analysis such as 18r DNA analysis of enteropneusts suggests that they are closer to the echinoderms than to the chordates.

    • Comment on echinoderm & chordate affinities of Balanoglossus.
    • Echinoderm affinities of Balanoglossus;-
    1. Adult resemblances:-
    Adult Balanoglossus & echinoderms are structurally quite different. So it is difficult to draw their phylogenetic relationship between themselves. However some resemblances are :-
    1. Enterocoelic origin of coelom.
    1. Heart vesicles & glomerulus of Balanoglossus are considered homologous to the dorsal sac & axial gland of echinoderm.
    1. Nervous system poorly developed & forms epidermal nerve plexus.
    2. Proteins & phosphagens present in Balanoglossus closely resemble those of echinoderms.
    3. Common habits, echological niches & remarkable power of regeneration.
    1. Larval resemblances:-
    Tornaria larva of Balanoglossus has striking, structural similarities with Bipinnaria larva of echinoderms.
    1. Small, pelagic, transparent & oval.
    1. Identical ciliated bands taking up a similar twisted course.
    1. Enterocoelic origin & similar development of coelom.
    1. Proboscis coelom of Tornaria is comparable to hydrocoel of echinoderm Dipleurula.
    2. Deuterostome & digestive tract is complete with mouth, anus & same parts.
    Comment;-
    Fell (1963) & others believe that their larval similarities are only because of convergent evolution due to same mode of habits & habitats.

    Chordate affinities of  Balanoglossus:-
    Inclusion of Balanoglossus under phylum chordata is not universally accepted.
    Among 3 fundamental chordate characteristics the nature of notochord is really questionable. Recent workers do not accept the notochordal nature of buccal diverticulum.
    The nervous system, in general, is typically non chordate type excepting the presence of lumen in collar nerve cord.
    The only important chordate feature is the gill slits.
    Affinities with Urochordata:-
    1. Similarity is present in the pharynx & branchial apparatus of both.
    1. Development of the central part of the nervous system is quite similar in both.
    Remarks:-
    The relationship becomes difficult to establish because the chordate nature of hemichordates itself is questionable.
    The resemblances are due to the fact that the hemichordates are very remotely connected with the central stalk from which urochordates have descended.
    Affinities with Cephalochordata:-
    1.  Similarity in the structure & function of branchial apparatus.
    1. Arrangement of coelomic sacs are similar.
    Remarks:-
    Presence of unsegmented muscle fibres in  Balanoglossus & dissimilarities in the developmental history stand as barriers to establish any close relationship between them.

    Comment;-
    Recent DNA analysis such as 18r DNA analysis of enteropneusts suggests that they are closer to the echinoderms than to the chordates.

    • Comment on the phylogenetic status of Peripatus./Discuss on the systematic position of Peripatus.
    Peripatus have both annelidan & arthropodan characteristics. Therefore they are regarded to be intermediate stage or connecting link between Annelida & Arthropoda.
    However they appear to be more closely allied to arthropods than to annelids. They may represent an offshoot from near the base of the arthropod line. Based on such phylogenetic considerations, Manton & other contemporary zoologists have included onychophorans (Peripatus) within the Arthropoda as a subphylum or class.
    But absence of exoskeleton, & jointed limbs & the presence of primary annelid characters in Onychophora create serious problems. Others claim that Peripatus is definitely an annelid.
    In fact, onychophorans are neither annelids nor arthropods but possess distinct characteristics of their own. Hence now a days, Onychophora is considered as a separate phylum.
    The modern view holds that Onychophora is not an evolutionary link between Annelida & Arthropoda but a distinct surviving branch. This is supported by the discovery of a mid-Cambrian fossil, Aysheaia  which closely resembles the modern Onychophora.
    • Onychophora exhibits both arthropod & annelid characters along with its own
    peculiarities- Explain.
    Arthropod characters of  Onychophora:-
    1. Presence of antennae.
    1. Body is covered with chitinous cuticle.
    1. Appendages are provided with claws.
    1. Jaws are modified appendages, provided with striped muscles.
    1. Locomotion by definite legs having definite musculature.
    2. Presence of haemocoel.
    1. Dorsal tubular heart with lateral ostia.
    1. Presence of tracheal respiratory system.
    1. Coelom reduced to small cavities that surround the excretory & reproductive ducts.
    1. Peculiar salivary gland supposed to be modified nephridia.
    2. Brain is large & typically arthropodan.
    3. General structures of the reproductive organs & development mainly arthropodan.
    Annelid characters of Onychophora;-
    1. Vermiform body with truncated extremities.
    1. Absence of true head.
    1. Outer body covering is cuticle, skin thin & flexible.
    1. Muscles separate in identical way.
    1.  locomotion slow & by peristalsis as in an earthworm.
    1. Simple & straight alimentary canal.
    2. Paired, segmentally arranged nephridia.
    1. Presence of cilia in excretory & reproductive ducts.
    1. Structure of eye same as in polychaetes.
    1. Slime & coxal glands correspond with similar glands of chaetopods.
    Onychophoran characteristics:-
    The following features are peculiar to Onychophora in which they differ from other phyla:-
    1. Body shows no or indistinct segmentation.
    2. Texture of skin. A rough cuticle covered with numerous velvety processes not known in other phyla.
    3. Antennae not homologous to the antennae of other arthropods.
    4. 3 segmented head of Peripatus shows a condition mid way between that of Annelida & Arthropoda.
    5. Restriction of jaws to a single pair. Movement of jaws is antero-posterior.
    6. Presence of non jointed legs with claws.
    7. Irregular distribution of spiracles or tracheal openings.
    8. 2 ventral nerve cords widely separated & without true ganglia.
    9. Structure of eye less complicated.
    10. Distribution of reproductive organs.

    • Write in details the arthropodan affinities of Peripatus.
    Arthropod characters of  Onychophora:-
    1. Presence of antennae.
    2. Body is covered with chitinous cuticle.
    3. Appendages are provided with claws.
    4. Jaws are modified appendages, provided with striped muscles.
    5. Locomotion by definite legs having definite musculature.
    6. Presence of haemocoel.
    7. Dorsal tubular heart with lateral ostia.
    8. Presence of tracheal respiratory system.
    9. Coelom reduced to small cavities that surround the excretory & reproductive ducts.
    10. Peculiar salivary gland supposed to be modified nephridia.
    11. Brain is large & typically arthropodan.
    12. General structures of the reproductive organs & development mainly arthropodan.

    • Make a note on the geographical distribution of Peripatus..
    The genus Peripatus is found restricted in various places of world such as South America & West Indies, South Africa, Australia, New Zealand, Tasmania, New Britain, , Malaya, Equatorial Africa & Chile.
    Some common forms of Peripatus are:-
    1. Neo-Peripatus:- (Peripatus):- West Indies & America
    2. Congo-Peripatus:- (Mesoperipatus Evans):- Congo & Caribbean islands.
    3. Eo-Peripatus:- (Eoperipatus Evans):- Malaya
    4. Capo-Peripatus:- (including Peripatopsis Pocock & Opisthopatus Purcell):- South Africa
    5. Melano-Peripatus;- ( Paraperipatus Willey):- New Britain.
    1. Austro-Peripatus:- (Peripatoides Pocock & Ooperipatus Dendy):- Australia, Tasmania, New Zealand.
    1. Chilio-Peripatus:- (Opisthopatus Bouvier), Chile.

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