10 SMITHSONIAN M ISCKl 1 AN IvOUS COl.l.KCTlONS VOL. Ill 



Tho contrast botwoon the oesophagus and tho upper jaw sac could 

 hardly he greater; and one can conclude that the interval between 

 these invaginations was an enormous period of time. In this interval 

 the open loops in the nerve chains would be closed ; and the fore- 

 brain would receive its most ancient addition. Judging from the 

 structure of the brain this is now the posterior section of the mid- 

 brain t^mid-brain 1): its small size (quite out of proportion with 



11 and 111) suggests its great antiquity. It would carry with it, too, 

 a [Xiir of cirri and a jxiir of eyes. At this time it would lie immedi- 

 ately behind the fore-brain (^ table J. stages 3 and 4). The process 

 iust envisaged would have two advantages for the animal: first, the 

 visceral nerves, as they had become, would thus constitute a system 

 of their own. apart from the "central" nerve cords, according with 

 their contrasting functions; and. second, the segmental ganglia behind 

 the new etYective mouth acquired agtiin direct connection with the 

 brain, thus rehahitating the "central" or rather the ventrolateral 

 nerve cords, as they then were. 



The first phcrytujiol invaijimUioii, comprising the compound upper 

 jaw s;ic suKlivided by the jaw jwds. appears to be mainly responsible 

 for the pharyngeal jxirt of the viscenil nervous system. The second 

 pharyngeal invagination, the lower jaw s;ic. appears to have added 

 only the anterior p;irt of the pharyng-eal nerves, as explained l^elow, 

 p. 18. The upper jaw sac. together with its bounding bifid lips above 

 and below \^l I and / //. fig. 2) is highly muscular ; and, except for the 

 muscles extending from it. it is boundevi laterally by the body cavity. 

 One of its great functions is its eversibility. allowing the fangs to be 

 shot out to seize prey. This is largely etTectet.1 by extroversion of the 

 main long s;\c beneath the jaw jxid. 



lleider was unable to trace the nervous system throughout; but he 

 believed his detaduxi jwrts to l>e continuous, as suggested in his 

 figure 10 t^see also fig. 5. acconnwnying this paper). Except for 

 the connections with the brain, all lie in the h}'i''*^dennis of the jaw sac, 

 and are lateral in position, from which we can infer that the ner\-e 

 corvls were still lateral and in the hypo«.iennis. The nerves connect- 

 ing with the oesophageal section have already been mentioned. Sup- 

 jx»sed to l>e coiuiected with these are the pair of main nervous centers 

 in the phar>-nx — Heider's "X-fonn bodies'* (^figs. 4 and 5; gx). In 

 plan these are irregularly quadrangailar. with concave sides and horn- 

 like angles, and they lie over the muscular posterior bases of the jaw 

 pads, on either side of the dividing rift and with their convexities 

 directed toward one another. Each consists of a nerve-fiber mass 

 coverevl with a layer of suKill ganglion cells. 



