92 G. H. PARKER 



therefore, seems certain that the lips are at best poor means of 

 nervous transmission from one half of the body to the otner. 

 Thus it appears that the scheme of nervous transmission pro- 

 posed by the Hertwigs ('79-80) and accepted with slight modi- 

 fications by Wolff ('04) and by Groselj ('09) fails in two funda- 

 mental particulars; first, it does not take into account direct con- 

 nections between the outer ectoderm, particularly that of the 

 column, and the deep-lying entodermic muscles, and, secondly, 

 such parts as the lips, which, according to this scheme, are an es- 

 sential portion of the connection between the ectodermic and en- 

 todermic nervous tracts, prove on examination to have almost 

 no capacity as nervous conducting organs. For these reasons, 

 as well as for the facts advanced in the preceding account, I 

 regard the scheme for nervous transmission advanced by the 

 Hertwigs as essentially untenable. Nervous transmission in ac- 

 tinians is accomplished in large part over tracts that penetrate 

 the mesogloea and is in no sense strictly limited to the basal 

 parts of the ectoderm and ihe entoderm. 



Such a plan, however, must not be interpreted as one of purely 

 diffuse transmission. There is ample evidence in certain parts 

 of Metridium at least of Specialized transmission. If the juice 

 from a crushed mussel (Mytilus edulis) is discharged on the 

 pedal disc or the column of Metridium, no response follows. If 

 on the other hand the juice is discharged on the tentacles, it 

 will induce the characteristic feeding movement of these organs 

 and a wide gaping of the esophagus. When discharged on the 

 lips, it is also followed by a wide opening of the esophagus. If a 

 solution of hydrochloric acid ^ in sea water is applied to the 

 column or to the tentacles, a retraction of the oral disc follows; 

 but if it is applied to the lips, a widening of the esophagus takes 

 place. Thus a stimulation of the tentacles by hydrochloric acid 

 induces a contraction of the longitudinal mesenteric muscles and 

 by mussel juice a contraction of the transverse mesenteric mus- 

 cles. Such diverse reactions from the same receptors must in- 

 volve a certain amount of internal differentiation in the trans- 

 mission mechanism which to my mind foreshadows in the primi- 

 tive neuromuscular mechanism of such animals as Metridium the 



