558 The Animal Kingdom 



The nervous system combines both vertebrate and invertebrate fea- 

 tures. A fibrous layer underlies the whole body surface. In the mid- 

 dorsal and midventral ridges this is thickened to form a dorsal and ventral 

 nerve trunk, respectively, which are connected by a nerve ring located at 

 the juncture of the trunk and collar. The dorsal nerve runs forward 

 into the collar to become the collar cord or central nervous system which 

 is separated from the epidermis. This at times has a central space, but 

 more often simply has isolated spaces. It has been shown that at least 

 part of this collar cord is formed in a manner similar to that of the 

 chordate brain, that is, by infolding of the ectoderm. 



The gill slits opening from the pharynx do not open directly to the 

 outside, rather they open into gill sacs which in turn open to the out- 

 side through gill pores. The excretory system is not well understood, 

 but it is believed that the glomerulus performs this important function. 



The sexes of Saccoglossus are separate. The saclike gonads, located 

 in the anterior portion of the trunk, open when the gametes are ripe to 

 the outside by minute pores. The eggs or sperm are discharged into the 

 water where fertilization occurs. Development proceeds through a larva 

 known as a tornaria larva (Fig. 185, C) which closely resembles the 

 bipinnaria larva of the starfish. 



