PHYLUM ARTHROPODA 



107 



antennules have tactile and chemical functions. On the ventral side of the 

 head are additional appendages belonging to the head, surrounding the mouth 

 and used for tasting, handling, and tearing food. These will be studied later. 



b) The thorax: Two longitudinal lines on the dorsal surface of the thorax 

 divide the carapace into a median cardiac region, covering the heart, and two 

 broad, curved, lateral branchial regions, which cover the chamber containing the 

 gills. The ventral side of the thorax bears five pairs of walking legs, of which 

 the anterior pair is modified into large pincers; some of the appendages in front 

 of the pincers also belong to the thorax. 



c) The abdomen: The abdomen is plainly divided into seven joints or seg- 

 ments, six of which bear appendages, used for swimming. The first pair of 

 abdominal appendages in male and female lobsters and in female crayfishes 

 and the first two pairs in male crayfishes are modified for sexual purposes. The 

 last pair of abdominal appendages is greatly broadened and forms with the last 

 abdominal segment a broad swimming fan. The last segment, or telson, has 

 no appendages and bears the anus on its median ventral surface. There is some 

 doubt that the telson is a true segment. 



d) Study of a typical segment and pair of appendages: As the abdominal 

 segments are more distinct and less modified than the other segments, one of 

 them, as the third or fourth, may be selected for study. Such a segment has 

 the general shape of a ring, as in the annelids. Its exoskeletal covering may 

 be divided into a convex dorsal portion, the tergum, a thin lateral plate, the 

 pleuron, extending free ventrally into a point, and a slender ventral bar, the 

 sternum. The region between the pleuron and the base of the appendage 

 receives the name of epimeron. Between successive segments occur thin arthro- 

 podial membranes, where the calcareous deposit is lacking; these are best seen 

 on the ventral surface between successive sterna, and they permit movement 

 of the segments upon one another. Examine the joints between segments at 

 the pleura; determine how they work by bending and straightening the abdomen. 



Between the base of the pleuron and the sternum of the segment is a round 

 area, into which the appendage is fastened by means of an arthropodial mem- 

 brane. Cut through this membrane and remove a complete appendage. It has 

 the following parts: It springs from the segment by a basal stem, the protopod. 

 This really consists of two joints, a very small basal piece, the coxopod, and a 

 long distal part, the basipod. From the basipod arise two flattened, many- 

 jointed plates. The outer of these is the exopod; the inner one, next the median 

 line, the endopod. Make a diagram of an ideal cross-section of the segment 

 with its pair of appendages. 



This type of appendage found on the abdominal segments is called the two- 

 forked or biramous appendage. It is supposed to be the primitive arthropod 

 appendage, all other kinds of appendages found in arthropods being derived 

 from it by modification. Theoretically, the original arthropod consisted of a 



