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Dissection of the following is required in A-level.
Aims for dissection
Mastering skills of dissection
Dissect the actual specimens and display the systems draw and label as many things as possible.
Precautions to good dissections
Always dissect along and not across. After dissection, draw.
DRAWING
Outline
A smooth outline no gaps/blockages, weary outlines earn you no marks
Use of single lines
These are used for drawing nerves.
Size of drawing
Use three quarters of the space provided.
The title and labeling
Every drawing must have a title obtained from the instructions. It must be specific, relevant and state the specimen e.g. if the specimen is called specimen X, stick to using X. Use label lines i.e. but not arrowss i.e.
Technical terms must be correctly spelt or interpreted e.g. nucleus vs mitochondrion vs mitochondria, ceacum vs ceaca.
Interpretation.
Vessels that suck blood to (arteries), vessels that drain blood from.
Magnification
The magnification must be stated which must have a bearing on the space given.
Use three-quarters of the space for drawing which must be X 1 or X 2 for a rat, and X1, X2, X3 for a toad, and X4-5 for a cockroach.
Neatness
– Take into consideration of everything in the drawing e.g. lines, labels, etc.
Time limits.
Maximum time spent on drawing = 15 minutes. The rest 45 minutes labeling correctly.
Adventurism should be encouraged.
In summary, marks are awarded in the following for any drawing represented at the dissection;
Magnification
Transport. Unless you are comparing 2 or more statements, do not write negative statements if given only specimen eg lacks tail.
Observation.
The following are important.
THE COCKROACH
Classification
Posterior dorsal view of a male cockroach
External feature
Anterior view of the head of a cockroach.
S.5 Biology practicals
Dissection of cockroach
2a. Spread out one pair of the wings, make an accurate size
Drawing of all the external features and label.
mode of life.
3a. State the features that can be used to differentiate between the
sexes of the specimen.
those structures used to identify the sexes in 3a) above.
5a. How many segments in each leg made of? Name of segments.
length of hind leg to fore leg.
7a. How many segments is the thorax made up of? Name them.
Solutions
A drawing of the external features of one pair of the wings of the cockroach.
Outer wing (Fore) Inner wing (Hind)
Rigid/stiff Flexible
Chitinous/thick Membrane/thin
Opaque Translucent
Smaller surface Large surface
Curved Flat
Oval shaped Triangular shaped.
Functions of parts of the alimentary canal and zoology.
Mouth
During chewing; the food is mixed with saliva, brought into the pre-oral cavity by the common salivary duct.
Salivary amylase converts starch receptacle.
Crop
Enzymes from the gut also come into the crop through the gizzard for digestion of some fats and proteins. The gizzard helps to masticate the food further by chitinours teeth and is filtered by the bottles.
Mid-gut (mesenteren).
The food, here mixes with secretions from the mid-gut lining and from the hepatic/digestive caroa which contains many enzymes e.g.
Ileum. The end of the mid gut whose lining absorbs digested food.
…..gut (color). The undigested food passes into the hind gut tat absorbs H20.
Rectum
NB Malphigian tubules are excretory organs attached to the ileum responsible for uric acid excretion but is not part of the alimentary canal.
The water in the uric acid can further be absorbed by the rectal glands in rectum to produce solid uric acid.
Mouth parts of cockroach under light microscope/hand lens after separation.
Functions of major mouth parts and adaptations.