ORIGIN AND STRUCTURE OF THE HEAD 121 



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 disappears. Thus VAN WVHE (1906) concludes: 

 "Amphioxus kann nicht horen ; er frisst mit dem linken Ohre 

 und hat infolgedessen den Mund verloren." 



Trimerism — ^[ACBRlDE (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 Balanoglossus 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 

 (I.e. 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 WVHE's pterygocoel, 

 in the metapleural folds, D.) ; the posterior pair is gradually 

 separated from the gut and, pari passu, devided into a 

 double 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 praemandibular 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 



