292 COLLEGE ZOOLOGY 



HABIT FORMATION. - - It has been shown by certain simple 

 experiments that crayfishes are able to learn habits and to modify 

 them. They learn by experience and modify their behavior slowly 

 or quickly, depending upon their familiarity with the situation. 

 One investigator has trained them to come to him for food. 

 (Holmes.) 



b. Crustacea in General 



(1) Distinguishing Features. - - The CRUSTACEA (Lat. crusta, 

 skin) are arthropods most of which live in the water and breathe 

 by means of gills. The body is divided into head, thorax, and 

 abdomen, or the head and thorax may be fused, forming a cephalo- 

 thorax. The head usually consists of five segments fused to- 

 gether; it bears two pairs of antennae (feelers), one pair of 

 mandibles (jaws), and two pairs of maxillae. The thorax bears 

 a variable number of appendages, some of which are usually 

 locomotory. The abdominal segments are generally narrow 

 and more mobile than those of the head and thorax; they bear 

 appendages which are often reduced in size. 



(2) Classification of the Crustacea. 1 - - The CRUSTACEA belong- 

 ing to Subclasses I-IV are often placed in one group and called 

 ENTOMOSTRACA. They are of small size, with a variable num- 

 ber of body segments, and usually no gastric mill in the stomach. 

 They are apparently more primitively organized than the mem- 

 bers of Subclass V, the MALACOSTRACA. Certain fossil animals, 

 called TRILOBITES (Fig. 209), are by many authorities included 

 with the CRUSTACEA. They have one pair of antenna?, and nu- 

 merous body segments, all of which bear biramous appendages. 



SUBCLASS I. BRANCHIOPODA. -- CRUSTACEA with an elongated 

 body, usually a carapace or shell, and many pairs of lobed, 

 foliaceous swimming feet. 



Order i. Phyllopoda. -- BRANCHIOPODA with from ten to 

 thirty pairs of leaf-like, swimming feet. Examples: 

 Branchipus, (Fig. 210, A), Artemia (Fig. 210, B). 



1 Somewhat simplified from Caiman in Lankester's Treatise on Zoology. 



