l86 AN INTRODUCTION TO ZOOLOGY 



Bilateral symmetry is already established at this early period: 

 the mesoblastic bands lie along what will become the longitudinal 

 axis of the future worm, and the mesomeres occupy the posterior 

 end. A gastmla is now formed by the invagination of the plate 

 of large cells (H) ; the edges of the cavity thus produced fold in 

 until only a slit remains (J). This slit is the blastopore (bp.), 

 and the cavity is the enteron, which later becomes part of the 

 alimentary canal. Soon the slit closes, except at one end where a 

 pore, the future mouth, remains. The three germ layers are at 

 this time quite distinct, and are well shown in a longitudinal 

 section of the gastrula (K). The large clear cells which invagi- 

 nated line the enteron, becoming the entoderm (en.)] the dome- 

 shaped epithelium of small cells covers the outer surface and repre- 

 sents the ectoderm (ec.) ; and between these two layers are the two 

 rows of mesoblastic bands which give rise to the mesoderm (mes.) 



(157)- 



The detailed history of these germ layers is too long and com- 

 plicated to be discussed in a book of this character. The develop- 

 ment of the mesodermal layers and ccelom should, however, be 

 mentioned. The mesoderm becomes separated into two layers 

 between which a cavity, the ccelom, is formed. The outer layer, 

 called the somatopleure, clings to the ectoderm, and gives rise to 

 the muscles of the body wall; the inner layer, called the splanch- 

 nopleure, remains attached to the enteron and gives rise to the 

 muscles of the alimentary canal. All other structures of meso- 

 dermal origin are derived from these two layers. After the es- 

 tablishment of the germ layers as described above, the embryo 

 elongates, and finally becomes vermiform, escaping from the 

 cocoon in about two or three weeks. 



Behavior. EXTERNAL STIMULI. The external stimuli that 

 have been most frequently employed in studying the behavior of 

 earthworms are those dealing with thigmotropism, chemotropism, 

 and phototropism (135, 145, 150, 152, 153, 155, 156). 



THIGMOTROPISM. Mechanical stimulation, if continuous and 

 not too strong, calls forth a positive reaction; the worms live 



