INVERTEBRATE Z0(")LOGY. 91 



Metamerism. Possesses true segments (somites or metam.eres). Are the 

 segments similar or dissimilar? What is the advantage of meta- 

 nieric differentiation? Is it greater or less here than in Lumbricus ? 

 Which is the higher V 



Body-regions {'=seginent-complexes). Three body-regions as in grass- 

 hopper ; first two fnsed, making (1) a cephalo-iltora.TC, composed of 

 immovable segments, and covered dorsally by a single shield-like 

 piece, the dorsal carapace, and (2) an abdomen of moveable seg- 

 ments. 



Cephalo-thorax. A transverse snture {cerviad suture) near the middle. 

 This marks division between liead and thorax. A median, anterior 

 process, the rostrum. The sides of the caraxmce are free and form 

 covering pieces for the gills (hranchiostegites). Break off one and 

 notice the gills underneath : also the lateral body- wall, delicate and 

 white, behind them. The gill chamber is thus external. Ventrally, 

 notice the appendages, five pairs of legs, of which the first is for 

 attack and defense, and not used in walking. The genital openings 

 are at the basal joints of a certain pair of legs, differing in the two 

 sexes. Compare different specimens. Where are they in the male •'' 

 in the female ? Find the mouth and note the large nixmber of ap- 

 pendages a])out it, used partly as jaws and partly to hold or taste the 

 food. 



Abdomen. Consists of seven segments, each with a large dorsal plate, 

 fergite. and a very narrow ventral piece, the sternite. The first five 

 segments bear small legs used for swimming (p)leopods). The ap- 

 pendages of the sixth segment are expanded, and form, with the 

 seventh segment, which is without api)endages, a five lobed tail. 

 How is this tail used ? In the spring the females attach the eggs and 

 young to the pleopods and protect them with the tail. The seventh 

 or final segment bears the anus. 



II. Appendages. 



There are nineteen pairs of appendages, each of which represents a somite. 

 Of these pairs, thirteen belong to the thorax and six to the abdomen. The 

 cephalo-thorax thus consists of thirteen somites and the abdomen of seven 

 (the last without appendages), giving a total of twenty somites. They are 



