Zoology

1. Dr. Arun Gaware
Assistant Professor,
Department of Zoology,
Shri Shivaji College, Motala, Dist. Buldhana

2.  While excretory organs of other chordates are coelomoducts or mesodermal
kidneys, those of Branchiostoma are entirely different.
 In Branchiostoma, the nephridia developes from the ectodermal cells and
have no relation with mesoderm.
 Thus, they are different from the kidney of vertebrates which are
mesodermal in origin.
 They closely resemble the protonephridia of flatworms or polychaete
annelids, thus providing a good example of parallel evolution.
 Some other organs and cells are also regarded to be excretory.

5. •A nephridium bears about 500 solenocytes. Each solenocyte is
nearly 50 µ long and consists of a tiny rounded nucleated cell body
and a long hollow stalk or tubule that opens into the lumen of
nephridial branch through a separate aperture.
•A long vibratile flagellum arising from a minute basal body in the
cell, extends through the tubule and projects freely into the lumen
of nephridium.
•The nephridia are richly supplied with blood vessels and the
solenocytes freely project into coelomic fluid.
• Excretion occurs by simple diffusion. Nitrogenous wastes are
extracted by solenocytes from blood and coelomic fluid,
discharged by nephridiopores into the atrium and passed out of
body through atriopore with the outgoing water current.
•As already mentioned, presence of protonephridia in amphioxus
is quite peculiar since they do not occur in any other chordate.

6. •2. Hafsehik's nephridium. A single long and straight tube,called the niphridium of
Hatschek, lies in the root of oral hood slightly towards left and ventral to notochord.
•Its anterior blind end lies somewhat ahead of the Hatschek's pit, but the posterior end
opens into the prebranchial sac of pharynx.
•It is ectodermal in origin and supplied blood by the dorsal aorta. Its structure is
basically like that of protonephridium already described.
• Its surface is covered by a number of solenocytes each surrounded by a coelomic sac
and extracting nitrogenous wastes.
3. Brown funnels. The two sac-like brown funnels are situated dorsally upon the
posterior end of pharynx, one on either side.
The narrow anterior end of each funnel opens into the epibranchial or dorsal coelomic
canal of its side, while the broad posterior end opens into the atrium.
Most workers consider the funnels to be excretory while some regard them to be
receptor organs.
4. Renal papillae. Groups of cells present on the floor of atrial cavity, form renal
papillae which are probably also excretory in function. Besides these, group of cells in
the atrial wall also serves the excretory function.
Moreover, inside the gonads, specially in testes, yellow masses are found containing
uric acid which are expelled with gametes also play important role in removal of
nitrogenous excretory metabolites.

7.  In Amphioxus is the two sexes are separate but
there is no sexual dimorphism as male and female
individuals look identical.
 Gonads. The adult has 26 to 27 pairs of similar
gonads, arranged metamerically in two rows, one
pair in each segment from 25 to 51.
 The gonads are situated ventro-laterally from the
middle of pharyngeal region upto the atriopore.
 They are clearly visible through the transparent
bodywall arranged in a linear series on either
ventro-lateral side beneath the myotomes.
 Gonads are simple hallow sacs, mesodermal in
origin and bulging conspicuously into the atrial
cavity.
 They are covered on the outer side by the
bodywall and on the inner side by the atrial
epithelium.
 Each gonad contains an outer secondary gonadial
cavity or gonocoel around and an inner primary
gonadial cavity surrounding a group of germ cells
which arise from its wall.

8.  Breeding season: Breeding season of the Amphioxus is the late spring and early
summer. The male and female emerge out of their burrows and the gamates are
liberated through the atriopore into the sea water where fertilization takes place.
 Thus fertilization is external.
 Gametes, Mature ovaries or testes can be identified only in sections because of the
different structure of spermatozoa and ova they contain. A testis presents a streaky
appearance due to presence of spermatozoa.
 The mature sperm of Branchiostoma, one of the smallest among chordates, is about
18µ in length.
 It consists of the usual nucleated head with acrosome, a middle piece and long tail. The
ovary contains ova which are large and somewhat "rounded cells each 0.1mm in
diameter and having a large nucleus.
 They are microiecithal or poor in yolk content (Fig. 41), Gonoducts are absent. Mature
gametes are discharged into the atrium by rupture or dehiscence of gonadial wall along
certain weaker spots called cicatrices which afterwards close. The gametes so
liberated, escape through atriopore with the outgoing water current.

10. •A similar quality in two or more animals
or things.
•Relationship of organisms, especially of
phylogenetic nature.
•Amphioxus possesses chordate as
well as many primitive non-chordate
characters.

11. Affinities with non-chordates:
•Development of coelom is enterocoelic in both.
•The heart is absent.
•Presence of segmentally arranged nephridia as present in polychaete annelids
•Gastrulation occurs by invagination.
Affinities with Urochordates:
•Presence of endostyle.
•Perforation of pharyngeal wall by numerous gill-slits which open into atrial cavity.
•Ciliary mode of feeding.
•Presence of atrium around the pharynx.
•Development of notochord and nerve cord is similar in both.
However, amphioxus differs from urochordates in following characters.
•Urochordates are sedentary but amphioxus is free living.
•Test is present in urochordates but absent in Amphioxus.
•Alimentary canal is straight in Amphioxus but U shaped in urochordates.
•Nephridia present in Amphioxus but are absent in Urochordates.
•The heart is absent in amphioxus but present in Urochordates.
•Gonoducts are absent in Amphioxus but present in Urochordates.
•Amphioxus is unisexual but ascidians are bisexual and asexual reproduction is also
reported in them.

12. Affinities with chordates:
Similarities
•Body wall is differentiated into epidermis and dermis .
•Mouth is ventrally situated.
•Coelom is similar to that of chordates.
•Notochord is situated on the dorsal side beneath the nerve chord.
•Presence of mid-gut diverticulum that can be compared with the liver.
•Pharyngeal gill-slits are present.
•Myotpmes are present.
•Development is similar to that of chordates
•Presence of hepatic system.
However, amphioxus differs from chordates in following characters.
•Absence of head, brain and cranium
•Presence of single layered epidermis.
•Ciliary mode of feeding.
•Heart is absent
•Presence of atrium around the pharynx
•Presence of colorless blood without respiratory pigment.
•Absence of special sense organs like eyes, ears and other sense organs.
•Absence of kidneys.

13. Conclusion. Branchiostoma possesses a peculiar admixture of primitive,
degenerate as well as specialized or secondary characters. As such, it can not be
placed in the direct line of evolution of chordates. It is regarded to be a generalized
and primitive type of chordate very close to the ancestral vertebrates. But, because
of its many differences, it is not included in the vertebrates. Instead it is placed in
an independent subphylum Cephalochordata.

14. Affinities of Agnatha:-
Affinities with Cyclostomata.
•Lack of jaws, exoskeleton, paired fins and gonoducts
•Persistent and continuous notochord.
•Segmented muscle blocks or myotomes.
•Numerous gill slits.
•Straight and simple alimentary canal.
•Dorsal and ventral roots of spinal nerves separate in lamprey.
The Ammocoete larva of lamprey (Cyclostomata) and
Branchiostoma show a striking similarity in many characters, such as :
•Elongated, slender fish-like body,
•Continuous dorsal median fin
•Mouth surrounded by an oral hood
•Ciliated gut
•Pharynx having endostyle and gill slits.
•Microphagus filter feeder