PHYLUM ARTHROPOD A 207 



in fundamental structure. Each consists of a terminal vesicle 

 which communicates with an external pore by a slender, greatly 

 coiled tubule. 



The digestive canal is largely ectodermal and is consequently 

 lined with chitin. The stomodaeum includes not only the 

 pharynx but also a dilated portion, modified for grinding, which 

 is called the stomach. Much of the intestine is proctodaeum. 

 The mesenteron is a relatively short region into which a paired 

 digestive gland, the hepatopancreas, empties. 



Sensory Organs. — Both antennae and antennules are tactile 

 organs, innervated from the brain, but in some instances the 

 antennae are the chief locomotor organs. Otocysts are found 

 only in the Malacostraca, where they occupy a position in the 

 protopodite of the antennules (Decapoda) or on the last abdom- 

 inal appendages (Schizopoda). Paired compound eyes are char- 

 acteristic of many crustaceans but there are some blind forms. 

 An unpaired X-shaped "nauplius eye" occurs in the larvae of 

 most crustaceans and is retained as the optic organ in the adults 

 of the lower Crustacea. 



Development. — The Crustacea typically pass through one or 

 more larval stages before reaching adult organization. When 

 direct development occurs, it has either resulted from a sup- 

 pression of larval stages, or these stages have been passed before 

 the embryo leaves the egg. The nauplius is one of the most 

 important as well as most characteristic of the larval stages of 

 Crustacea, for it is almost universal among the lower orders. 

 This larva is composed of three segments bearing three pairs of 

 appendages. The foremost of these appendages are simple and 

 later develop into the antennules, while the second and third are 

 biramous and in later development form the antennae and 

 mandibles respectively. There is a single unpaired eye. 



In a characteristic instance, as in the development of Cyclops, 

 the nauplius after its first molt becomes a metanauplius bearing 

 the rudiments of three pairs of appendages in addition to those 

 found in the nauplius. These additional rudiments represent 

 the two pairs of maxillae and one pair of maxillipeds. A pair 

 of very rudimentary thoracic appendages also make their 

 appearance on the metanauplius. In each of three successive 

 molts, the Cyclops larva acquires an additional pair of these 

 stumphke rudiments of the thoracic legs, so with the close of the 

 larval period there are four pairs of the thoracic rudiments, and 



