Zoology

COMMENTS: • Intestine starts after 15th segment . It shows beaded appearence. Section is triploblastic with organ grade of construction and true coelom . • Intestine is divisible into 3 regions :- (i) Pre - typhlosolar region (15th to 26th segment). (ii) Typhlosolar region (from 26th segment to rectum)  (iii) Post typhlosolar region (rectum).  Typhlosolar region has median dorsal internal fold . • Body wall is composed of thin cuticle , columnar epidermis , thin circular muscle layer and thick longitudinal muscle layer, parietal coelomic epithelial layer. • Intestine is composed of outer visceral coelomic epithelial layer , intermediate longitudinal and circular muscle layer and it's lining is made up of endodermal columnar epithelial ( layer) cells. In typhlosolar region epithelial cells are raised to from typhlosole .  • Typhlosole is a median internal fold of dorsal wall having chloragogen cells and a capillary blood vessel. • Typhlosole increase the surface ar

COMMENTS: •The section shows organ grade of construction with triploblastic layeration and true coelom . • Oesophagus terminates into prominent oval, hard, thick-walled and distinct gizzard lying in 8th and 9th segment. • Body wall is composed of a thin cuticular muscle layer, thick longitudinal muscle bundle, parietal epithelial layer and a thick circular septum , thin cuticular and columnar epithelial epidermis . • Gizzard is a hard muscular organ, composed of visceral coelomic epithelial layer, thick circular muscle fiber, and lined internally by columnar epithelial cells and thin cuticle .  • Gizzard is a masticatory apparatus and breaks food into small particles. • Gizzard can at once be recognized by thick continuous muscle fibre and various blood vessel . • Section also contains dorsal and ventral vessels , ventral nerve cord , integumentary nephridia and lateral nerve cord , and lateral oesophageal vessels . • Few muscle strands are also seen in the section.

 COMMENTS: • The section shows organ grade of construction , triploblastic layeration and true coelom . Pharynx is clearly seen. Pharynx is wide pear shaped ; thick walled muscle chamber. • Pharyngeal cavity is dorsoventrally compressed . • Cuticle is thin, non-cellular, double-layered, indescent and made up of protein , collagen , gelatin and polysaccharides. • Body wall is composed of cuticle , epidermis , musculature and parietal epithelial layer . • Epidemis consist of single layered distinct columnar cells with gland cells , supporting cells, basal cells and receptor cells. • Musculature comprises outer continuous circular muscle fibre and inner longitudinal muscle fibres cut in bundles. • Pharynx contains visceral epithelial layer , pharyngeal gland cells , musculo-vascular tissue and lumen of pharynx. • Lumen is divided by incomplete horizontal shelf into upper salivary chamber and lower conducting or ventral chamber . Salivary chamber contains ciliated pharyngeal

Comments: It is the common larva for annelida and mollusca. • In the development of Nereis , trochophore larva is formed after gastrulation . • Larva is conical or top shaped , microscopic and fast swimmer . • Anterior end contains apical sensory ongan having tuft of cilia . Nerve ganglion is present below it. • Digestive system is complete consisting of lateral mouth , rounded sacular stomach , intestine and anus . •There are three defined ciliated bands for swimming/locomotion through ciliary beats -  (i) Pre-oral ciliated band or prototroch (ii) Post oral ciliated band or metatroch . (iii) Telotroch •Internally larva contains coelom , larval nephridium (protonephridium) , mesoderm , eye spot and larval muscles.   Food groove is present between prototroch and metatroch . Ciliated organs guide food towards food groove which chanelise food into mouth. In some cases, neurotroch is present at posterior end and acts as sensory organ .   •After metamorphosis , trochophore