Question

Which one of the following categories of animals, is correctly described with no single exception in it?
A. All bony fishes have four pairs of gills and an operculum on each side.
B. All sponges are marine and have collared cells.
C. All mammals are viviparous and possess diaphragm for breathing.
D. All reptiles possess scales, have a three-chambered heart and are cold blooded (poikilothermic).

Answer 1

Hint: Animals are classified based on the similarity in them. Previously it was only based on morphology but now it is based on genetic analysis. Because of that reason a lot of organisms show varied characters than the others.

Complete answer:
Let us look at each of the options given in the question.
Bony fishes or Osteichthyes have a bony endoskeleton and overlapping scales on the outer surface. They have four pairs of gills and they are covered by an operculum on both the sides. Gills serve as the respiratory organs for fishes and helps in gaseous exchange.
Next is the sponges. Majority of the sponges are marine but some of the sponges live in freshwater habitat as well. So, this option is wrong as the question says all sponges are marine but that is not the case.
The next option says that all mammals are viviparous and they possess diaphragm. It can be noted that there are egg laying mammals or monotremes such as Platypus and Echidna which are considered to be oviparous and hence, this option is wrong as well.
The last option states that all reptiles have three chambered hearts, cold blooded and possess scales. There are some snakes that do not possess scales. A crocodile which is a reptile does not have three chambered hearts; it has a four chambered heart.

From this information we can conclude that option A is the right answer.

Note:
It can be noted that even though a character is unique to a phylum, class etc., there are few organisms in the same taxon that do not show the same character. This is very well seen in the highest organism of a taxon where it resembles the next higher group and in lowest organism of a taxon which shows characters of a previous phylum etc.