NO. 3 INSECT lIEAll SXODCRASS 9 



were withdrawn into the hody where they became the rudiments of a 

 central nervous system. Other sensory cells, remaining at the surface 

 but sending processes inward to the buried cells, formed the peripheral 

 sensory system. This anterior differentiation of sensory and con- 

 ductive tissues opened still other possibilities of cephalization, which 

 have led to the development of the brain and all the com])lex sense 

 organs located on the head in higher animals. 



It is difficult to establish, by concrete example, the contention that 

 the change in the position of the mouth resulted from a change in the 

 manner of locomotion, but it is indisputable that the ancestors of the 

 worms and the arthropods must have assumed the crawling habit at 

 some stage in their evolution. The chaetopod annelids, in their em- 

 bryonic development, arrive at a first larval stage known as a trocho- 

 phore (fig. 7 D), which is a free-swimming creature with well differen- 

 tiated anterior and posterior poles, and a dorsal and a ventral surface, 

 with the mouth situated anteriorly in the latter. If dorsiventrality is 

 to be attributed to a creeping mode of locomotion, then there must 

 be some stage omitted between that represented by the free-swimming 

 planula, and that of the free-swimming trochophore, because there is 

 no evident reason, otherwise, why two forms having the same mode 

 of life should have an organization so different. The trochophore is 

 without doubt a specialized larval form modified secondarily for a 

 swimming habit. It can not, therefore, be taken to represent an an- 

 cestral form of the worms ; but it is the only free-living creature that 

 shows us the beginning of the worm organization, and its structure 

 can certainly be traced into that of the arthropods. 



The annelid trochophore is typically ovate in shape with the larger 

 end forward (fig. /D), or rather, upward, since the creature floats 

 upright in the water, but the side in which the mouth (Mth) is located 

 is called the ventral surface because it becomes the under surface of 

 the mature worm. The mouth lies a little below the middle of the 

 body, and the anus (Aji) is situated at the posterior pole. The body 

 is surrounded by several bands of vibratile cilia. The principal band 

 (b), comprising usually two rows of cilia, is situated on the widest 

 part of the body and just before the mouth. It divides the animal into 

 a preoral, or prostornial, region (Pst), and into a postoral, or iiictas- 

 tomial, region (Mst). A second band of cilia (c) is generally present 

 a short distance behind the mouth, and sometimes there is a third, 

 preanal band (G, d) near the posterior end, which sets off a terminal 

 circumanal region, or pcriproct (Ppt). At the anterior end of the 

 body there is a central tuft of tactile hairs (G, a), a pair of small 

 lateral tentacles (Tl), and one or more simple eye spots (0). 



