234 Prof. M'lutosh's N'otes from the 



tierve-trunks wholly hyporlermal. The latter arrangement 

 of the ventral cords is that characteristic of most Polychttts 

 — yet about ten families have their great ventral nerve- 

 tvuiiks enclosed by the muscular tissues of the body-wail, 

 besides the basemeut-layer, hypoderm, and cuticle, showing 

 how uncertain any single factor is in the classification of this 

 group. In the AVcliiannelid. Protodrilus the nervous system 

 agrees with that in Owenia and Myriochele in being mainly 

 hypodermic, aud in Saccocirrus and Sternaspis (though this 

 is not a Polychtet) the cerebral ganglion is similarly situated 

 and the ventral nerve-cord is not segmented into ganglia. 

 They contrast thus with the Nemerteans, in which the 

 cephalic ganglia are internal and the longitudinal cords either 

 enveloped in the muscular walls of the body or entirely 

 within them. It is further interesting, in couipariug 

 the Nemerteans with the Polychaets, that no Polyclnet 

 possesses the proportionally large nerve-supply to any organ 

 — a suppl}'', moreover, more bulky in its distiibution than in 

 its origin, and which undergoes remarkable changes of form, 

 both in contraction and dilatation — as that of the Nemerteau 

 proboscis. Hence its lattice-like arraiigemeut gave rise 

 to the term " elastic layer " in the early memoirs. This 

 feature is as noteworthy as the passage of the proboscis 

 between the dorsal and ventral commissures of the cephalic 

 ganglia. Some consider that this arrangement of the nerves 

 causes it to be an organ of sensation ; but it is oftea thrown 

 off when brought into contact with foreign bodies, and, 

 though renewed, its functions for the interval are in 

 abeyance. In the Ammocharidse under consideration wliat 

 appears closely allied to nervous matter is distributed as 

 a continuous layer beneath the hypoderm of the gullet — a 

 condition much more primitive than the elaborate system of 

 the Nemertean proboscis or than the proboscis of a typical 

 Tolychvet such as Nereift. 



In glancing at the literature of the subject, it is found that 

 the acute and accomplished Claparede, familiar as he was 

 with the ordinary nervous system of the Polychaeta, failed to 

 find the central nervous system in Owenia " qu'il m^a ete 

 parfaitement impossible d'en trouver la moindre trace chez 

 V Owenia fusifortms sur les coupes d'individus conserves ^^ ■^ 

 and he even had difficulty in discriminating the ventral cord 

 in the fresh animal. Yet he had described and figured 



&' 



a 



similar condition to that of Owenia in Telepsavus costarum, 

 one of the Chsetopteridse, in which the central nervous 



* Annel. Sedeiit. p, 129. 



