OSTEICHTHYES-BONY FISH

GENERAL CHARACTERS
Inhabit all sorts of water-fresh,brackish or salt warm or cold water.
Body spindle-shaped and streamlined.
Fins both median and paired ,supported by fin rays of cartilage or bone.
Tail usually homocercal.
Skin with mucous gland,usually with embedded dermal scales of 3 types ganoid,cycloid or ctenoid.
Placoid abescent.
Endoskeleton chiefly of bone.
Claspers absent.
Mouth terminal or subterminal.

BRANCHIOSTOMA-GAMETES

Mature ovaries or testes can be identified only in sections because of the different structure of spermatozoa and ova they contain. The testes present a streaky appearance due to presence of spermatozoa. The mature sperm of branchiostoma one of the smallest among chordates in length. It consist of the nucleated head with acrosome a middle piece and a long tail. The ovary contain ova which are large and some what rounded cells each 0.i mm in diameter and having a large nucleus. They are microlecithal or poor in yolk content. Gonoducts are abesenr. Mature gametes are discharged into the atrium by rupture or dehiscence of gonadial wall along certain waeker spots called cicatrices which after wards close. The gametes so liberated escape through atriopore with the outgoing water current.

BRANCHIOSTOMA-REPRODUCTIVE SYSTEM

In amphioxus the two sexes are separate but there is no sexual dimorphism as male and female individuals look identical.
GONADS-The adult has 26 to27 pair of similar gonads arranged metamerically in two rows,one pair in each segment from 25 to 51. The gonads are situated ventro-laterally from middle of pharyngeal region up to the atriopore. They are visible through the transparent bodywall arranged in a linear series . Gonads are simple hollow sacs mesodermal in origin and bulginginto atrial cavity. They are covered on the outer side by the bodywall and on the inner side by the atrial epithelium. Gonads contain an outer secondary gonadial cavity around an inner primary gonadial cavity surrounding a group of germ cells which arises from its wall.
DEVELOPMENT-Fertilization and development take place externally in sea water .

BRANCHIOSTOMA-FREE NERVE ENDINGS

These are sensitive to internal stimuli such as contractions of muscles.The exteroreceptors there are also present some enteroreceptorsin the form of free nerve ending in muscles.

BRANCHIOSTOMA-SENSORY CELLS AND PAPILLAE

Sensory cells scattered all over the epidermis,especially on the dorsal side. Groups of sensory cells or papillae are present on oral cirri and velar tentacles. These are chaemoreceptors and tactile organs.

BRANCHIOSTOMA-INFUNDIBULAR ORGAN

It is located at the floor of cerebral ventricle . It consists of a patch of tall ,columnar and strongly ciliated epitheliar cells. The exact function of infundibular orgain is uncertain. The various function assigned are detection of pressure in the cerebro-spinal fluid perceiving the shadow cast by cephalic pigment spot or neurosecretion.

BRANCHIOSTOMA-KOLLICKER'S PIT

It is a depression of ciliated ectodermal cells on the roof in the anterior region of cerebral vesicle. The area where larval neuropore closes when the adult neural tube is formed. It lacks sensory cells but it is considered anolfacrory chaemoreceptor due to its position corresponding with the single nostril of cyclostomes.