414 CEPHALOCHORDA, AMPHIOXUS. 



dorsal nerves, and the many-rooted, motor, ventral nerves 

 of higher Vertebrates. But the dorsal nerves of Amphioxus 

 supply the muscles as well as the skin, so that they must be 

 partly motor. Furthermore, there is no connection between 

 the two sets, and the dorsal nerves have no ganglia. Nor 

 are there any sympathetic ganglia. 



The anterior region of the nerve cord is said to exhibit some histolo- 

 gical, though no morphological distinctiveness. With it the following 

 structures are associated : 



(a) Slightly to the left side there is a ciliated pit, often called olfactory. 

 The development of this is interesting. The cavity of the medullary 

 tube opens at first to the exterior by an anterior aperture, the neuropore. 

 Later, an invagination of the ectoderm takes place at this point, and 

 carries the neuropore in with it. This invagination forms the olfactory 

 pit ; it at first opens into the neural tube by the persistent neuropore ; 

 later this closes, and the pit becomes a mere blind sac. This invagina- 

 tion may perhaps correspond with the ciliated duct of the sub-neural gland 

 of Tunicates, and so with part of the hypophysis of other Vertebrates. 



(b) At the end of the nerve cord there is a pigment spot, sometimes 

 called an eye spot. There are no true eyes. 



(c) On the roof of the mouth there opens a small sac, the pre-oral pit, 

 which may have a tasting or smelling function. It arises in development 

 from the left of two pouches which grow out anteriorly from the gut of 

 the embryo. The right of these pouches forms the head cavity of the 

 adult, so that ontogenetically the pre-oral pit is the aborted head cavity 

 of the left side. This is, however, only one of many explanations of the 

 organ. 



It is likely that the most important sensory structures of the adult are 

 the sensitive cells of the epidermis. 



We may connect the feeble development of sense organs with the 

 almost sedentary habit. 



Alimentary System. 



The true mouth or velum lies well within the projecting 

 pre-oral hood with its fringe of cirri. In the larva this hood 

 is absent, and the mouth is flush with the surface. 



The mouth opens into the pharynx, which, like it, is 

 richly ciliated. The pharynx, like that of Tunicates, and 

 indeed of Fishes also, is modified for respiration (Fig. 133, 

 Ph.). Its walls are perforated by numerous gill slits on each 

 side, and between these lie supporting bars and arches, 

 alternately split and unsplit. 



Along the mid-dorsal and mid-ventral lines there are 

 grooves respectively called hyper- and hypobranchial. The 

 latter is comparable to the endostyle of Ascidians. 



