ZOO202 Midterm Notes

Lesson 18 · ZOO 202 · Mid Term Notes

Q1. Why are protochordates not classified as true Chordata?

Like the remaining subphylum of the chordates, the Vertebrata, the protochordates have; A hollow dorsal nerve cord Gill slits, A stiff supporting rod, The notochord, The forerunner of the backbone. The protochordates differ chiefly from the vertebrates in not having a backbone. The organisms belonging to the Protochordata are generally known as the lower chordates. They don’t form a “proper” taxonomic group and are only classified as such for convenience purposes. However, they do form a major division of Chordata. They are also known as Acraniata because they lack a true skull. They are generally found in marine water. Their body is bilaterally symmetrical, triploblastic, and coelomated. At a certain stage of their lives, their body develops a long, rod-like structure for support called notochord. They exhibit organ system level of organization. Eg., Herdmania, Amphioxus.

(…..First classification of protochordata…..) Hemichordata

Q2. What is Hemichordata (Tunicates & Amphioxus)?

Marine, Solitarily Deuterostomate animals with a body divided into three regions: proboscis, collar, and trunk; coelom divided into three cavities Body is divided into proboscis, collar, and trunk.

Primitive notochord restricted to proboscis only, thus called stomochord.

True coelom present Straight or U shaped gut with anus Nervous system normally diffuse and variable Open circulatory system Blood has no color and corpuscles. Excretory organGlomerulus Filter feeding Four classesEnteropneusta, Pterobranchia, Planctosphaeroidea, Graptolita Eg., Cephalodiscus, Rhabdopeura

Q3. ExplainClass I – Enteropneusta (Acorn worms) ?

General Characters

Marine worms Usually range in size between 10 and 40 cm More than 70 species most occupy U-shaped burrows in sandy and muddy substrates between the limits of high and low tides. Proboscis is a short, conical projecton at the worm’s anterior end. A ring like collar is posterior to the proboscis An elongate trunk is the third division of the body A ciliated epidermis and gland cells cover acorn worms. The mouth is located ventrally between the proboscis and the collar.

A variable number of pharyngeal slits, from a few to several hundred, are positioned laterally on the trunk.

Pharyngeal slits are openings between the anterior region of the digestive tract, called the pharynx, and the outside of the body.