WITH NOTES ON THE WEST INDIAN SPECIES. 



315 



IV. Collar Nerve-cord and Roots. 



Spengel does uot regard the collar nerve-cord of the Enteropneusta as the central 

 nervous system but as being only a part of it which has become closed in. This is a 

 highly important view, and it is again with satisfaction that I find myself in agreement 

 with Spengel. He defines the collar nerve-cord (Mon. p. 705) as "die Einsenkung des 

 auf den Kragen entfallenden Theiles des dorsalen Nervenstammes, der in seiner ganzen 

 Lange vom Grunde der Eichel bis in die Nahe des Afters, bereits vorher nicht nur 

 angelegt, sondern in alien seinen charakteristischen Zligen ausgebildet ist." 



The centralisation of the nervoiis system of the Enteropneusta has not proceeded 

 far. There is concentration along the middle line on the dorsal or sensory side of the 

 body and another concentration along the middle line on the ventral or locomotor side 

 of the body\ The dorsal concentration or dorsal nerve-cord of the trunk passes directly 

 into the ventral wall of the medullary tube in the collar region and at the junction 

 of medullary tube and dorsal cord, i.e. at the posterior edge of the collar a circular 

 commissure passes round to unite with the anterior end of the ventral nerve-cord. 



Bateson pointed out that the dorsal side of the medullary tube of the collar was 

 the sensory side and received afferent fibres through the so-called dorsal roots or at 

 the two ends of the cord in those forms which do not possess roots ; and that the 

 ventral side of the medullary tube from which efferent fibres pass into the muscles, is 

 the motor side. The conclusion he came to was practically the only one possible at 

 that time, namely, that the roots of the collar cord of Enteropneusta "are to be 

 regarded as the homologues of the dorsal roots of other Chordata." 



"In Balanoglossus-," says Bateson (188G loc. cit. p. .558), "we see in the trunk the • 

 cord still in the skin, in the collar the cord delaminated'*, and at the ends of this cord 

 the process of invagination commencing and leading to the presence of a lumen." 



This quotation shows that Bateson was alive to the fact that the collar cord is 

 only a local differentiation of the dorsal cord as a whole. This fact is still more clearly 

 expressed by Morgan (Journ. Morph. Vol. ix. 1894, see p. 74) in the following words, 

 which I heartily endorse : — " We see in Balanoglossus that the invaginated dorsal nerve- 

 cord can correspond only to the anterior end of the nerve-cord of Amphioxus, and 

 that the superficial dorsal nerve-path, stretching through the gill region thence to the 

 end of the body, must be the homologue of the remainder of the nerve-cord of 

 Amphioxus." 



1 The ingenious method of homologising the reverse sides of the body in Vertebrates and Invertebrates 

 by employing the terms "neural" and " liaemal " instead of "dorsal" and "ventral" is a gigantesque example 

 of a petitio principii. 



- That is to say Bui. l-oivalevskii, 



3 In Bal. koivalevskii the collar nerve-cord arises in a peculiar manner akin to delamination. In Tornaria 

 as shown by Morgan and in regenerating Ptychodera flava as shown on Plate XXVI. it arises by fusion of 

 medullary folds. 



