ALIMENT AR Y AND RESPIRA TOR Y S YSTEMS. 4 1 5 



the region of the two first pairs of dorsals, and behind this 

 they divide up into many minute fibres just as they leave 

 the nerve-cord. The two sets of nerves are compared 

 respectively to the single-rooted sensory dorsal nerves, and 

 to the many-rooted motor ventral nerves of higher Ver- 

 tebrates. But the dorsal nerves of Amphioxus supply the 

 transverse muscles as well as the skin, so that they are pro- 

 bably partly motor. Furthermore, there is no connection 

 between the two sets, and the dorsal nerves have no 

 ganglia, except in so far as these are represented by aggre- 

 gations 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 : — (1) 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. 

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

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

 invagination of the ectoderm takes place at this point, and carries the 

 neuropoie 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 blind sac. This invagination 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, 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 seems to arise 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. 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 



