?34 



Prof. M'lntosli's Xotcs from the 



nci-vc-tmnks whollv liypodermal. The latter arraTigement 

 of the ventral cords is that characteristic of most Polychats 

 yet ahout ten families have their «;reat ventral nerve- 

 trunks enclosed by the iiiuscnlar tissues of tlie body-wall, 

 besides the basement-layer, hypoderni, and cuticle, showing? 

 liow uncertain any siuii^le factor is in the classification of this 

 group. In the .Vrehiannelid Prulodrihis the nervous system 

 agrees with that in Oivcnia and Myriocliele in being mainly 

 hypodermic, and in Saccocirrus and Stcniaspis (though this 

 is not a Polychict) the cerebral ganglion is similarly situated 

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

 Tiiey contrast thus with the Nemerteans, iu which the 

 cephalic ganglia are internal and the longitudinal cords either 

 enveloped in tiie muscular walls of the body or entirely 

 within them. It is further interesting, in comparing 

 the Nemerteans with the Polycluets, that no Polychiet 

 possesses the proportionally large nerve-supply to any organ 

 — a supply, moreover, more bulky in its distribution than in 

 its origu), and which undergoes remarkable changes of form, 

 both in contraction and dilatation — as that of the Neinertean 

 proboscis. Hence its lattice-like arrangement gave rise 

 to the term "elastic layer '^ iu the early memoirs. This 

 feature is as noteworthy as the passage of the proboscis 

 between the dorsal and ventral commissures of tiie cephalic 

 ganglia. Some consider that this arrangement of the nerves 

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

 off \>hen brought into contact Avith foreign bodies, and, 

 though renewed, its functions for the interval are in 

 abeyance. In the AmmocharidcC under cousideration what 

 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 j)roboscis of a typical 

 Polycluet such as Nereis. 



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

 the acute and a(Com[)lishcd Clypaiede, familiar as he was 

 Avith the ordinary nervous system of the Polyehaeta, failed to 

 find the central nervous system in Oiuenia " qu"*]! m'a ^t6 

 j)arfaiteraent impossible d'en trouver la moindre trace chez 

 V Oweniu fu.nfonnis sur les coupes d'individus conserves^'*, 

 and he even had difficulty in discriminating the ventral cord 

 in tiie fresh animal. Yet he had described and figured a 

 similar condition to that of Owenia in Telepsavus costaruni, 

 one of the Chaetopteridac, in which the central nervous 



• Anntn. S(5dent. p. 129. 



