Which is true?

Options

A. All living members of class cyclostomata are parasites on some fishes
B. There are about 2000 species in the class osteichthyes
C. Ciona belongs to subphylum cephalo chordata
D. Arthropods are diploblastic animals

Answer

To determine which statement is true, let's analyze each option step by step:

1. **Statement 1:** "All living members of class Cyclostomata are parasites on some fishes."
- Cyclostomata is a class within the phylum Chordata, specifically under the group Agnatha, which includes jawless fishes such as lampreys and hagfishes.
- These organisms are known to be ectoparasites, meaning they attach to the outside of larger fish and feed on their blood or bodily fluids.
- Therefore, this statement is **true**.

2. **Statement 2:** "There are about 2000 species in the class Osteichthyes."
- Osteichthyes refers to bony fishes, which is one of the largest classes in the vertebrate subphylum.
- In reality, there are approximately 28,000 species of bony fishes, making this statement **false**.

3. **Statement 3:** "Ciona belongs to the cephalocordata."
- Ciona is a genus that belongs to the subphylum Urochordata, not Cephalocordata. In Urochordates, the notochord is present in the tail region during the larval stage.
- Thus, this statement is also **false**.

4. **Statement 4:** "Arthropods are diploblastic animals."
- Arthropods are characterized by having three germ layers: ectoderm, mesoderm, and endoderm, which makes them triploblastic.
- Therefore, this statement is **false**.

After analyzing all the statements, the only true statement is:

**Correct Answer:** Statement 1 - "All living members of class Cyclostomata are parasites on some fishes."

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