Zoology 10th Edition by Stephen Miller,John Harley

Exercise 5

Two types of glands are found in vertebrates. Endocrine glands secrete hormones into the blood for distribution throughout the animal's body and bind to those cells or tissue that have specific receptors for the hormone. Exocrine glands secrete their products into a duct and the effect is where the duct empties.
Why do we know more about the anatomy and physiology of vertebrate endocrine systems as compared to invertebrate endocrine systems?
FIGURE 25.7 Vertebrate Glands with and without Ducts. ( a ) An endocrine gland, such as the thyroid, secretes hormones into the extracellular fluid. From there, the hormones pass into blood vessels and travel throughout the body. ( b ) An exocrine gland, such as a sudoriferous (sweat) gland, secretes material (sweat) into a duct that leads to a body surface.