Correct Option
The correct option isThyroid gland
Explanation
Glands in the animal body are primarily classified based on the method of secretion transport:
- Exocrine Glands: Possess ducts (tubes) to transport secretions to specific body surfaces or cavities.
- Endocrine Glands: Are "ductless" and release secretions (hormones) directly into the bloodstream or interstitial fluid to be transported to target organs.
Option Analysis
- Salivary gland Salivary gland: Incorrect. Salivary glands are exocrine glands. They secrete saliva into the oral cavity through specific ducts (e.g., Stensen's duct for the parotid gland).
- Sweat gland Sweat gland: Incorrect. Sweat glands (sudoriferous glands) are exocrine glands located in the skin. They release sweat onto the skin surface through ducts for thermoregulation and excretion.
- Sebaceous gland Sebaceous gland: Incorrect. Sebaceous glands are exocrine glands usually associated with hair follicles. They secrete sebum (oil) through ducts to lubricate the skin and hair.
- Thyroid gland Thyroid gland: Correct. The thyroid gland is a major endocrine gland located in the neck. It lacks ducts and secretes hormones, such as thyroxine ($T_4$) and triiodothyronine ($T_3$), directly into the circulatory system to regulate metabolism.
Key Takeaway: The thyroid gland is an endocrine (ductless) gland that secretes hormones into the blood, whereas salivary, sweat, and sebaceous glands are exocrine glands that utilize ducts to release secretions onto epithelial surfaces.