55 
visceral cleft, the spiracle of fishes, and called the “tremo- 
stoma”. Even a nephridium has been found to belong to it, 
just as to the other gill-slits, viz. the unpaired HATSCHEK’s 
nephridium (VAN WYHE, 1914, p. 67). The antimer of the 
mouth is found then in the club-shaped gland which 
originates as an entodermal pouch at the right side of the 
body, in front of the series of gill-slits of that side, and 
afterwards acquires an opening to the exterior. A few 
years before WILLEY (1891, p. 225) had shown reason for 
considering the club-shaped gland as a gill slit belonging 
to the right side. During further development the whole 
organ diíisappears. Thus VAN WYHE (1906) concludes: 
“Amphioxus kann nicht hören; er frisst mit dem linken Ohre 
und hat infolgedessen den Mund verloren.” 
Trimerism — MACBRIDE (1898), finally, in agreement with 
BATESON’s (1885) and MASTERMAN's (1897) attempts to 
explain the structure of the Chordates by a derivation 
from forms like Balanogiossus and the so called Diplo- 
chorda (MASTERMAN), tries to find traces of an original 
trimeric arrangement of the coelomic cavities. These arise 
in the larva of Balanoglossus as one anterior unpaired 
and two posterior paired entoderm pouches or, if this 
be not strictly the case, yet soon after show a similar 
arrangement. In the same way, according to MACBRIDE 
(lc. p. 606), “the mesoderm originates in Amphioxus as a 
series of true gut pouches, viz. one anterior unpaired pouch 
and two pairs of lateral pouches” Of these the first divides 
to form the two head cavities; the anterior pair give rise 
to the first pair of myotomes, and, in addition, to two long 
canals extending back ventrally (VAN WYHE's pterygocoel, 
in the metapleural folds, D.) ; the posterior pair are gradually 
separated from the gut and, pari passu, devided into a series 
of myotomes. 
Thus the anterior unpaired pouch is represented by the 
head-cavities, — according to VAN WYHE only by the right 
one, compared by him to the premandibular cavities of 
Craniates — which are homologized by MACBRIDE to the 
proboscis-cavity of Balanoglossus. The first pair of paired 
pouches is represented by VAN WYHE's mandibular seg- 
ments which, according to MACBRIDE, show a certain 
independence from the following segments in the way in 
which they are separated from the gut and in their prolonged 
communication with the latter. They are called by MACBRIDE 
