520 CEPHALOCHORDA 



there is no connection between the two sets, and the dorsal 

 nerves have no gangUa, except in so far as these are repre- 

 sented by aggregations of nerve nuclei. Nor are there any 

 sympathetic ganglia. 



The nervous system of the lancelet is thus very divergent from what 

 is typical for Vertebrates : — (i) A brain is almost undeveloped ; (2) the 

 ventral roots far outnumber the dorsal roots ; (3) the two sets of roots 

 do not unite ; (4) the dorsal nerves are partly motor ; (5) there are no 

 spinal ganglia ; (6) there are no sympathetic ganglia. 



The anterior region of the nerve-cord exhibits some histological dis- 

 tinctiveness ; and with it the following structures are associated : — 



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

 olfactory. It arises from an ectodermic invagination in the position 

 of the neuropore or original anterior opening of the nerve-cord. Below 

 this there is a minute diverticulum from the front dorsal wall of the 

 nerve -cord. 



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

 called an eye-spot. There are no true eyes, but numerous regularly 

 arranged pigment spots on each side of the spinal cord appear to be 

 optic. 



(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 is likely that the most important sensory structures of the adult 

 are the sensitive cells of the epidermis. The feeble development of 

 sense organs may be associated with the almost sedentary habit. 



Alimentary and respiratory systems. — The true mouth 

 lies within the projecting pre-oral hood. It is surrounded 

 by a membrane called the velum, and is fringed by twelve 

 velar tentacles, which must not be confused with the 

 external cirri. In the larva the 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. 296, 

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

 each side, and between these lie supporting bars alter- 

 nately split and unsplit at their lower ends. 



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

 grooves, respectively called hyper- and hypo-branchial. 

 The latter is comparable to the endostyle of Ascidians, 

 by which name it is often called. As in Ascidians, two 

 ciliated bands — the peripharyngeal bands — encircle the 

 anterior part of the pharynx. 



The water-current which enters the mouth is, as in 



