Question

Which among the following does not have placoid scales ?
A. Rays
B. Silver fish
C. Skates
D. Sharks

Answer 1

Hint: It belongs to Phylum Arthropoda , which is the largest phylum of Animals which includes insects .

Complete answer:
1. Silver fish is not a fish it belongs to Phylum Arthropoda , which includes insects .
2. Placoid scales are only present in class Chondrichthyes. Which belong to Phylum chordata.
3. Chondrichthyes belong to subphylum vertebrata in superclass pisces( bear fins ).
4. 4. Chondrichthyes are marine animals with streamlined body and have cartilaginous
endoskeleton. Mouth is located ventrally. Notochord is persistent throughout life.
5. Gill slits are separate and without operculum ( gill cover ) . The skin is tough containing minute placoid scales. Teeth are modified placoid scales which are backwardly directed . Their jaws are very powerful and they are predaceous.
6. Example: Scoliodon ( Dog fish ) , pristis ( Saw fish ) Rays , Skates Shark etc .

So correct answer is (B).

Note:
1. Some more characters of Chondrichthyes are :
a. Due to absence of bladder , they have to swim constantly to avoid sinking.
b. Some of them have electric organs ( e.g., Torpedo ) and some possess poison sting ( e.g.,
Trygon )
c. They are cold blooded ( poikilothermous ) animals i.e. , they lack the capacity to regulate
their body temperature.
d. Sexes are separate. In males pelvic fins bear claspers .
e. They have internal fertilization and many of them are viviparous .