ORIGIN AND STRUCTURE OF THE HEAD 



127 



then proves to be right. It seems to me less probable, 

 however, that this mouth should be a secondary one in 

 regard to that of Craniates. The latter is found again by 

 VAN WYHE in the praeoral or HATSCHEK's pit. He was 

 led to this conclusion on the same a priori ground as is 

 adduced by Neal (see above). According to VAN WYHE 

 (1907, p. 69) this change, not of function but, of organ 

 would find its cause in the spiral movement of the larva 

 during its pelagic life. However plausible this may be 

 rendered by VAN WVHE, from the point of view of my 

 theory it seems unnecessary to look for any other forerun- 

 ner of the present mouth than the neuropore. After 



the loss of the neuropore, which in young stages still per- 

 haps acts as a mouth, one of the gill- 

 slits has taken over its function, as ex- 

 pressed by a modification of VAN 

 Wyhe's sentence, proposed by me 

 (1913) in a former paper: "er hat den 

 Mund (den Neuroporus!) verloren und 

 frisst infolgedessen mit einer Kiemen- 

 spalte". 



First pair 0/ gill-pouches. — If, how- 

 ever, as argued on page 85 of this 

 treatise, it may a priori be expected 

 more or less that the first pair of gill- 

 slits would have broken through at the 

 limit of the prostomium and the first 

 segment, then we might expect to find 

 a pair of gill-pouches in front of the 

 mouth and the club-shaped gland and 

 in front of the mandibular segment. 

 As such, I believe, must we consider, 

 as Hatschek (1892, p. 144) originally 

 did, both the anterior entodermal di- 

 verticula. In figs. 23 and 24, after 

 Hatschek, we see them originate in 

 a strictly bilateral way as evaginations 



Fig. 24. Anterior end 

 of the same stage 

 of Amphioxus as fig. 

 23, seen from the 

 dorsal side and sho- 

 wing, besides the first 

 pair of gill-pouches 

 ("anterior entoderm 

 pocicets"), the rostral 

 prolongations of the 

 first pair of somites. 



after Hatschek, 1882, 

 fig. 52. 



of the entoderm just in front of the mandibular somites, 



exactly where and as we should expect a first pair of 



gill-pouches. One of them, the left, still acquires an opening 



to the exterior, the praeoral pit which, according to HATSCHEK 



(1892, p. 144), is innervated in a similar way as the mouth 



