CRUSTACEANS 409 



less powerful extensor muscles, lying above the gut, straighten it 

 for another downward movement. 



The Central Nevvous System (fig. 249) consists of a nerve- 

 ring surrounding the gullet, and a double ventral cord upon which 

 numerous pairs of ganglia are developed. The upper part of the 

 nerve-loop is thickened into a pair of brain or cerebral ganglia, 

 by which the important sense organs of the head are supplied. 

 The general arrangement conforms to that described in the Cock- 

 roach (see p. 349). 



Organs of Sense are well developed, and many of them are 

 in the form of specialized bristles or seta, the soft axes of 

 which are in communication with the skin by means of vertical 

 canals which perforate the hard exoskeleton. Many of these 

 setae minister to the sense of touch, and this is especially true of 

 those found on the antennules and antennae. Organs of taste 

 are not definitely recognized, though they probably exist on some 

 of the appendages of the mouth. Certain peculiar spatula-shaped 

 setae present on the external branches of the antennules have to 

 do with smell. 



The extremely interesting auditory organs consist of two pear- 

 shaped sacs, one of which is lodged in the basal joint of each 

 antennule, and opens by a slit to the exterior. These sacs are 

 really in-pushings of the skin, and they contain numerous special- 

 ized auditory setae, and also grains of sand which have been 

 introduced from the exterior. The chief interest attaching to 

 these organs lies in the fact that they correspond to a stage in 

 the development of more complex structures, such, e.g., as the 

 membranous labyrinths of Vertebrates, which start as pits in the 

 .skin (see p. 56). 



The visual organs are in the form of two compound eyes 

 situated on the front of the head near the antennules, and borne 

 on stalks. 



Development. The Lobster passes through a metamorphosis 

 in the course of its life-history, for it hatches out as a larva, which 

 differs in many ways from the adult. 



Crustacea, of which the Lobster has been described as a type, 

 may be defined as aquatic Arthropods possessing two pairs of 

 feelers (antennules and antennae), and breathing organs (when such 

 are present) in the form of gills. The appendages are typically 

 forked. The class is subdivided in the following way: 



