266 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



with van ^Vijhe, Hatschek, aud Willey, that the first myotome iu Amphi- 

 oxus is situated behind the first two pairs of uerves, and also that the 

 "anterior head cavities" of Amphiosus represent the somatic elements 

 of the anterior segment in Craniota. From this point of view, the 

 diflferent fates which the two cavities in Amphioxus undergo, as well as 

 the loss of musculature, is to be regarded as coeuogenetic. The dorsal 

 nerves in both forms are exclusively sensor in correlation with the loss 

 of splanchnic musculature.^ The unpaired olfactory of Amphioxus is to 

 be compared with the long persisting median connection of the neural 

 tube and olfactory plate in Squalus {lohus olfactorius impar, Kupflfer). 

 Xo somatic musculature and no ventral nerves are developed. I regard 

 the cerebral vesicle of Amphioxus, since it is limited posteriorly by the 

 tuberculum posterius (Kupffer), as homologous with the primary fore- 

 brain of Squalus, and therefore as the neuromeric element of the first 

 segment.^ Whether or not the visceral cleft of this segment is repre- 

 sented in the "anterior entodermic diverticula" (anterior head cavities), 

 I am not able to assert with any degree of positiveness. Fiirbringer's 

 discovery appeal's to favor this view. 



In the second segment a well developed myotome and ventral nerve 

 develop. Iu both forms the dorsal nerve of this segment appears to be 

 exclusively sensor. In Myxinoids, however, the dorsal nerve (ophthal- 

 micus profundus) has motor fibres, and it appears to me not improbable 

 that such will be found in its homologue in Amphioxus.^ If the vis- 

 ceral-cleft element in this segment is not represented in the " anterior 

 head cavities," this may be assumed to have disappeared phylogeneti- 

 cally. All the components of the third segment are present, viz. somatic 

 and splanchnic musculature, dorsal and ventral nerves, and visceral clefts. 



1 The homology of the olfactorius (I) with the first paired nerve of Amphioxus has 

 already been asserted by Owen (1866). The first paired nerve of Amphioxus accord- 

 ing to Owsjannikow (1866) and Rabl ('89) is homologous with trigeminus; with r. 

 orbito nasalis or r. I. trigemini (Huxley, 1874) ; with opticus (Schneider, '79) ; with 

 part of trigeminus (Rohon, 1881, and Krause, 1888); with ophth. prof. trig. (Hat- 

 schek, '92) ; and with nervus apicis (van Wijhe, '93). 



2 KupfEer ('93) homologizes the cerebral vesicle of Amphioxus with the Vorhirn 

 (Vorderhirn and Mittelhirn) of Craniota. 



3 The second paired nerve of Amphioxus has previously been homologized 

 with part of trigeminus by J. Miiller (1842), W. Muller (1875), and Krause (1888) ; 

 with trigeminus by Goodsir (1841) ; with trigeminus and vagus by Quatrefages 

 (1845) and Owen (1866) ; with facialis by Owsjannikow (1867) ; with opticus by 

 Basse (1876) ; with part of trigeminus and with facialis by Rohon (1881) ; with 

 acustico-facialis by Rabl ('89) ; with trigeminus exclusive of ophth. profundus by 

 Hatschek ('92) ; and with ophth. profundus by van Wijlie ('93). 



