MORPHOLOGY OF ANIMALS 



III. ORGAN SYSTEMS 



Organ systems are two or more organs associated in a 

 common function. We recognize the following systems: 

 (i) Integumentary, (2) Skeletal, (3) Motor, (4) Alimen- 

 tary, (5) Respiratory, (6) Circulatory, (7) Excretory, (8) 

 Reproductive, (9) Nervous, (10) Sensory. 

 1. Integumentary System 



(^) Epidermis. In all animals the outer covering of the 

 body consists of a layer of epithelial cells, derived from 

 the ectoderm and known as the epidermis. Beneath this 

 layer a basement 7nembrane is present, which in some 

 animals is thick and serves for protection and support 

 (Cnidaria, Platoda). This epidermis is frequently ciliated 

 and it always contains gland and sensory cells and in 

 addition may contain nerve and muscle cells as well as 

 stinging cells (Cnidaria). In some animals the epidermis, 

 which in these cases is called hypodermis^ secretes on its 

 outer surface a cuticular covering which may be a thin 

 and flexible membrane or cuticle (hydroids, trematodes, 

 cestodes, annelids, rotifers), or it may be thick and flexible 

 (nemathelminths) or dense and inflexible except at the 

 joints (arthropods). In other cases the epidermis secretes 

 skeletal structures in certain regions only, thus giving rise 

 to calcareous shells (corals, moUusks, brachiopods). In 

 arthropods this epidermal secretion is particularly dense 

 and tough and is known as chitin; it may become calcified 

 in certain portions. In mollusks the superficial epidermis 

 remains naked except in a certain region, the embryonic 

 shell-gland, where it first secretes a cuticular covering and 

 then forms beneath this a dense calcareous layer, the shell ; 



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