The statement "skin is the main excretory organ" is incorrect because in all chordates kidneys act as main excretory organ.

The submaxillary duct is also known as the Wharton's duct opens into the floor of the mouth. They are the major ducts of the submaxillary (submandibular gland) and help in the transfer of saliva from the submaxillary gland to the opening under the tongue.

Sweat glands are coiled and tubular in structure. Coiled part of this gland is called as secretory unit. Around secretory unit, myoepithelial cells are present. When they contract sweat comes out from sweat gland. This secretion is called as sweat. 

The skin has sweat glands that excrete perspiration (salt, water, and nitrogenous wastes) through its pores. 

Skin serves in excretion of waste material through the process of sweating. Large intestine serves to reabsorb water and essential minerals from the undigested food and to store it till it is excreted at the anus. Lungs serve in excretion via expiration. The liver serves in the formation of urea which is then excreted by kidneys. Kidneys are the organs of urine formation. This makes option B correct answer as large intestine only stores the undigested material; it does not excrete it directly. 

Evaporation or the loss of water from the body due to heat is called as perspiration. The heat present in the surrounding causes water to be lost from the body in the form of sweat through the sweat glands on the skin. The water is deposited on the surface of the organism's body leading to a cooling effect. Thus the correct answer is option A.

Sweat glands secrete a fluid called sweat that contains water and dissolved salts of Calcium, Sodium, Magnesium etc Thus, it helps in excreting water and salts to some extent by a process called sweating or perspiration. 

Since terrestrial animals do not have automatic access to either fresh or salt water, they must regulate water content in other ways, balancing off gains and losses.

Ammonia which is the major form of nitrogenous waste generated in the body is converted urea in the liver and excreted through the urine by the kidneys. Thus, the correct option is C.

The accessory excretory organs are:-

(A) Spleen - Primarily acts as blood filter. 

Metabolic wastes are substances left over from a metabolic process which cannot be used by the organisms and must, therefore, be excreted. This includes nitrogen compounds. The nitrogen compounds through which excess nitrogen is eliminated from the organism are called nitrogenous wastes. They are ammonia, urea, uric acid, and creatinine. 

The large intestine is the part of the digestive system. Whereas the excretory system of humans consists of two kidneys, two ureters, one urinary bladder, and one urethra. Urine is found in almost all excretory system parts(little or more).

The epidermis is composed of 4 or 5 layers depending on the region of skin being considered. The Malpighian layer (stratum malpighii) is defined as both the stratum basale and stratum spinosum. In spinous layer (stratum spinosum), keratinocytes become connected through desmosomes and start to produce lamellar bodies, from within the golgi, enriched in polar lipids, glycosphingolipids, free sterols, phospholipids and catabolic enzymes. Basal/germinal layer (stratum basale/germinativum) is composed of proliferating and non-proliferating keratinocytes, attached to the basement membrane by hemidesmosomes. Melanocytes are present, connected to numerous keratinocytes in this and other strata through dendrites. Merkel cells are also found in the stratum basale with large numbers in touch-sensitive sites such as the fingertips and lips. 

The sweat on the surface of the skin is evaporated by the body heat. As the water is evaporated by heat released from the body, the body cools off. Each gram of water on the body surface absorbs about 0.58 calories of energy before evaporating, which is a significant source of heat loss.

Certain invertebrates like some molluscs (Unio, Limnaea, etc.) and some echinoderms (e.g., Asterias) excrete excess amino acids as such. These animals are called aminotelic and their mode of excretion is called aminotelism.

A flame cell is a specialized excretory cell found in the platyhelminthes like planaria. They  function like a kidney, removing waste materials by the process of osmoregulation.

(A) Nephridia - Leech

Organisms which excretes amino acids are called aminotelic. Organisms like Asterias, Uni, Limnaea etc have an aminotelic excretory system. Oniscus is a land crustacean which excretes uric acid and falls into uricotelic category.

Collecting ducts are the larger tubes, each receiving the collecting tubules of several nephrons. Collecting duct does not form a part of the uriniferous tubule.

The major mode of excretion in birds is uricotelism. Uricotelism is the excretion of uric acid and the animals which excrete uric acid are called uricotelic. For example, birds. Uric acid crystals are non-toxic and almost insoluble in water. Hence, these can be retained in the body for a considerable time. So, the correct answer is 'Aminotelism is the major mode of excretion of birds'.

The limbic system is basic to motivation, emotion, and reward processes. It controls emotional expression through the hypothalamus, which has control over the body's emotional responses systems. The thalamus in the limbic system (' leopard brain') signals the brain thus we get to feel satisfaction for each mouthful of food. The acid and semi-digested fats in the duodenum trigger the enterogastricreflex – the duodenum sends inhibitory signals to the stomach by way of the enteric nervous system, and sends signals to the medulla that inhibit the vagal nuclei, thus reducing vagal stimulation of the stomach. Association cortex is the cerebral cortex outside the primary areas. It is essential for mental functions that are more complex than detecting basic dimensions of sensory stimulation, for which primary sensory areas appear to be necessary. So the correct option is 'A−2,B−3,C−4,D−1'.

Following are the correct statements about the excretion of earthworm: 

A - Malpighian tubule