Lamprey : Ammocoete : Larva Whole Mount
- It is one of the stages in the development of lamprey. Egg develops into ammocoete larva, which is a tiny transparent creature in the beginning and later on becomes opaque and 170 mm long. It lies buried in mud.
- General body form is eel-like. Body divisible into head, trunk and tail.
- Mouth or buccal cavity is surrounded by upper and lower lips, having a number of buccal tentacles or oral cirri.
- Velum is found at the posterior end of the buccal cavity followed by the pharynx.
- Pharynx has 7 pairs of gill-slits. Gill arches and gill lamellae lie in the wall of these pouches. Ventrally pharynx contains double strand of mucus-secreting cells called as endostyle which forms thyroid gland of the adult.
- Head contains median nostril, eyes on sides and an auditory organ and brain.
- Larva contains median fin which forms continuous dorsal, caudal and ventral fins.
- Nerve cord with anterior brain divisions extends antero-posterioriy along the notochord. Notochord extends along the entire length of the body. Myotomes are segmentally arranged. Heart lies ventrally posterior to the pharynx and has 3 chambers.
- Digestive system after pharynx consists of oesophagus, wider intestine, anal opening, liver and gall bladder.
- Ammocoete larva shows intermediary characters between Cephalochordata and Cyclostomata.
Identification :- The larva contains seven pairs of gill-slits and all above characters, hence it is Ammocoete larva.
Lamprey : Ammocoete Larva : T.S. Passing Through Branchial Region
- T.S. passing through branchial region shows distinct gill lamellae along with nerve cord and notochord, etc.
- Body wall is composed of epidermis, dermis and muscle layer. The muscles comprise of thick myotomes, separated by myocommata.
- Nerve cord lies just between supporting tissue and notochord.
- Notochord is surrounded by the notochordal sheath and has vacuolated chordal cells. Below notochord is wide-Iumened pharynx. It contains several gill lamellae supported by lateral muscles.
- On dorsal side of the pharynx is dorsal aorta and mid-dorsal ciliated ridge, while on ventral side is thyroid involution.
- Below thyroid involution bundles of mid-ventral muscles are seen.
- Pharynx also contains velum, ciliated bands and endostyle. Pharynx performs both nutritive and respiratory functions.
- Endostyle in larval condition secretes mucus but during metamorphosis it develops into thyroid gland, which contains iodine vesicles.
Identification :- Since the larva contains mid-dorsal ciliated ridge and all above characters, hence it is T.S. of Ammocoete larva passing through branchial region.
Lamprey : Ammocoete Larva : T.S. Passing Through Intestine
- T.S. passing through the intestine shows usual parts of the body wall, nerve cord, notochord, muscles and intestine.
- Body wall is composed of epidermis, dermis and myotomes, which are especially developed on dorsal side.
- Supporting tissue and nerve cord lie close together.
- Vacuolated notochord is surrounded by notochordal sheath.
- Intestine is ventrally situated. It is lined by endodermal columnar cells.
- On ventral side intestinal epithelium is raised to form typhlosole, which increases absorptive surface.
- Sub-vertebral blood sinus is found below the notochord.
- Intestine is surrounded by coelomic cavity lined by coelomic epithelium.
Identification : Since this section has typhlosole in intestine and all above characters, hence it is T.S. of Ammocoete larva passing through intestine.
Petromyzon : T.S. Passing Through Branchial Region
- T.S. passing through branchial region shows body-wall layers, prominent gill lamellae along with muscles, nerve cord, !lotochord and other visceral structures.
- Body wall is composed of epidermis made up of stratified squamous epithelium, which is sometimes covered with polyhedral cells, dermis and muscle layer. Epidermal cells may be mucus-secreting cells, granular cells and club-shaped cells.
- Dermis is made up of a dense connective tissue of compact fibres.
- Muscle layer comprises of thick myotome bundles and myosepta or myocommata.
- Cut notochord is composed of vacuolated notochordal cells covered by notochordal sheath.
- Above notochord is nerve cord and adipose tissue, which forms fat column.
- Dorsal aorta is found below notochord followed dorsoventrally by oesophagus, respiratory tube and ventral aorta.
- Anterior cardinal sinus and efferent branchial artery are found on the sides of dorsal aorta in tandem position.
- Afferent branchial artery is found on each side of the ventral aorta.
- Two gills consisting of gill lamellae enclosing gill pouch occupy large space in the ventral half of the section.
Identification : Since this section contains large pharynx with gill pouches and all above structures, hence it is T.S. of Petromyzon passing through branchial region.
Petromyzon : T.S. Passing Through Trunk Region
- This section can be at once recognized by the presence of various visceral organs, containing body-wall layers, nerve cord, notochord, kidney, gonads and intestine, etc.
- Body wall comprises of polyhedral cells, stratified squamous epithelium, dermis and muscle layer. Epidermal cells consist of mucus secreting cells, granular cells and club-shaped cells.
- Dermis is made of dense connective tissue of compact fibres.
- Myotomes separated by myocommata are very distinct.
- Anterior dorsal fin is supported by cartilaginous radials arranged in a single series. Some fin muscles are also seen in the section.
- Vacuolated notochord is composed by notochordal tissue covered by notochordal sheath, which with its basement membrane forms elastic intema and a thin black elastic extema. Nerve cord is found above notochord.
- Posterior cardinal veins, dorsal aorta below notochord, and two kidneys with mesonephric duct.
- Gonad (ovary) is suspended from dorsal body wall by mesovarium. The intestine with spiral valve is also seen below ovary. Coelom is present.
Identification : Since this section contains visceral organs and all above features, hence it is T.S. of
Petromyzon passing through trunk.
Petromyzon : T.S. Passing Through Tail Region
- Section is smaller in outline.
- Body wall is composed of epidermis, dermis and muscles. (For details of body wall layers, see T.S. Petromyzon passing through branchial region).
- Dorsal and ventral lobes of the caudal fin are supported by cartilaginous radials.
- Spinal cord, notochord, caudal artery, and caudal vein are seen in the middle of the section.
- Myotomes and myosepta are distinct. Identification : Series the section contains caudal fin and above features, hence it is T.S. passing through the tail region of Petromyzon
- Book of Practical Zoology VERTEBRATE by S.S.LAL – Rastogi Publications.