72 



and (VIII) in the above list. (V) is, I think, almost equally certain ; and, as Masterman states, 

 it lies at the base of the glandular epithelium of the proboscis (PI. IV, figs. 36, 42). I have 

 not been able to tracé it with complete certainty round the dorsal or lateral margins of the 

 proboscis, although figs. 151 — 146 (PI. XII) appear to indicate that this is really its mode 

 of connexion with the central nervous system. I think that Masterman is right in describing 

 the operculum as being supplied with a nerve-layer, although I have not indicated it in most 

 of my figures, and I am not certain of its Communications with other ]:)arts of the nervous 

 system. But it may be remarked that Cephalodisctis resembles Balanoglossus in possessing a 

 general nerve-plexus in its epidermis, and that it is only the thicker parts of that plexus which 

 can be recognised as definite nerve-tracts. It might be going too far to say that the plexus is 

 universal in Cephalodiscits, but it is certainly present in many of the thicker parts of the 

 epidermis, while its existence is further indicated by the fact that the nerve-tracts commonly 

 fade away gradually at their edges. 



The following account of the nervous system, which is based on my own observations, 

 besides giving a general account of the whole system, refers specially to the parts described 

 by Masterman and numbered (UI), (VI) and (VII) in the above list. 



The nervous system of Cephalodisciis is very strongly developed, and I emphasize this 

 fact because it has recently been asserted by de Selys Longchamps (04, p. i 1 3) that it is 

 "peu développé" or even "rudimentaire". This is very far from being the case, the central 

 nervous system in particular consisting of a large mass of nerve-tissue. It has been pointed 

 out by Morgan (94, p. 72) that in retaining its central nervous system in the outer epidermis, 

 Cephalodisats must be regarded as more archaic than its relation Balanoglossus. 



The central nervous system has essentiall)- the same characters in all the species, even 

 the curiously reduced male individuals of C. sibogac forming no exception to this statement. 

 It gives rise to a great thickening of the epidermis in the dorsal region of the collar, extending 

 thence on to the posterior part of the proboscis. It is shewn in more or less sagittal sections in 

 PI. IV, figs. 42, 41 (C. dodccalophtcs), figs. 34^ — 36 (C. Icvinsciii), fig. 37 (C. gracilis), PI. VTII, 

 *ï&- 93 (^- sibogae, neuter) and PI. VII, fig. 79 [C. sibogac, male); while PI. X, figs. 112 — 118, 

 referring to C. levinseni, illustrate its relations as seen in frontal sections. The greater part of 

 the nervous mass consists of a conspicuous layer of fibrillar material, resting on the basement- 

 'membrane; while ganglion-cells may be clearly distinguished, in certain parts of the organ, 

 among the bases of the epithelial ectoderm-cells. Thus in the original of fig. 42, some half dozen 

 ganglion cells occupy the epidermic swelling which in the figure appears to project into the 

 mass of fibrils near the posterior end of the brain. 



In his latest paper on Cephalodiscvs (03, p. 717) Masterman describes what he terms 

 the "ectodermal pit", a crescentic groove running transversely across the region of the central 

 nervous .system, and representing the line of division between the proboscis and the collar. 

 The proboscis-pores are said to open at the outer ends of the crescent. I have on various 

 occasions noticed transverse grooves corresponding to the "ectodermal pit", but I think that 

 M.ASTERMAN is wrong in describing it as a definite structure. It appears to me to be the result 

 of the varying positions of the anterior end of the body and of the varying states of contraction 



