266 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
with van Wijhe, Hatschek, and Willey, that the first myotome in Amphi- 
oxus is situated behind the first two pairs of nerves, and also that the 
“anterior head cavities” of Amphioxus represent the somatic elements 
of the anterior segment in Craniota. From this point of view, the 
different fates which the two cavities in Amphioxus undergo, as well as 
the loss of musculature, is to be regarded as coonogenetic. The dorsal 
nerves in both forms are exclusively sensor in correlation with the loss 
of splanchnic musculature.t The unpaired olfactory of Amphioxus is to 
be compared with the long persisting median connection of the neural 
tube and olfactory plate in Squalus (lobus olfactorius impar, Kupffer). 
No 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), 
Tam not able to assert with any degree of’ positiveness. Fürbringer’s 
discovery appears to favor this view. 
In the second segment a well developed myotome and ventral nerve 
develop. In both forms the dorsal nerye 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, 798). 
2 Kupffer (93) homologizes the cerebral vesicle of Amphioxus with the Vorhirn 
(Vorderhirn and Mittelhirn) of Craniota. 
8 The second paired nerve of Amphioxus has previously been homologized 
with part of trigeminus by J. Müller (1842), W. Müller (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 
Hasse (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 Wijhe (93). 
