312 



ENTEROPNEUSTA FROM THE SOUTH PACIFIC, 



/ therefore suggest that the orifice of the praeoral pit of the larva of Amphioxus 

 rejoresents the opening of tlie coeloiii into the end-sac of the Enteropneusta ; the end-sac 

 and its external orifice are represented in Amphioxus by Kollikers olfactory pit which 

 coincides in its point of origin with the pre-existing neuropore, which hence opens into its 

 hase^; the coelomic portion of the anterior trematic complex is therefore still existent in 

 Amphioxus, hut it is separated from its terminal portion (end-sac) concurrently with 

 the forward extension of the notocJiord in the same way as the mouth has been dis- 

 sociated from the neuropore^. 



Hatschek described, as of mesodermal origin, a subchordal praeoral tube on the 

 left side of the larva of Amphioxus. This has been called Hatschek's nephridium, 

 and its opening into the anterior buccal portion of the pharynx was described and 

 figured by Lankester and Willeyl MacBride (loc. cit.) has recently found that at an 

 early stage this tube is in open primary communication with the somite which he 

 has called the left collar-cavity, and in fact that it arises as a canalicular extension 

 of the hollow stalk which connects the left collar-cavity with the archenteron. It 

 only occurs in the larva, is lost during the metamorphosis, and is probably a vestigial 

 structure. 



The collar-canals of the Enteropneusta may be said to open into the pharynx 

 through the mediation of the first gill-pouch and of the first gill-cleft (cf. PI. XXXII. 

 Fig. 52). The inference is obvious that Hatschek's nephridium is an unpaired vestige 

 of the excretory canals of the collar region. 



I have already (see pp. 273 and 280) compared the vestigial truncal canals of 

 Spengelia with the atrio-coelomic funnels (Lankester) of Amphioxus. 



1 This is in complete accordance with the view which I have expressed on a former occasion, that Kolliker's 

 olfactory pit represents the neuro-hypophysial canal of the Ascidian larva. By the epidermal invagination (at a 

 late stage) which produces Kolliker's olfactory pit the neuropore is carried inwards at its base, and no longer 

 opens flush with the surface of the body. In this way the neuropore acquires a new quality, namely, it 

 becomes the cerebral opening of the olfactory pit or neuro-hypophysis. Thus in Amphioxus, the neuropore 

 and the inner or cerebral opening of the neuro-hypophysis coincide. That there are two structures involved 

 is indicated by the fact that the neuropore exists for a long time in the absence of a neuro-hypophysis 

 (olfactory pit). As described by me in Ciona (loc. cit.) it appears that a large portion of the duct of the 

 adult subneural gland is derived from a secondary evagination of the wall of the buccal siphon at the lips of 

 the primary branchial or buccal orifice of the neuro-hypophysis (see Quart. Journ. Micro. Sci. Vol. xxxv. 

 pp. 305 — 306). In this way the primary opening of the neuro-hypophysis into the mouth is carried inwards 

 just as the neuropore in Amphioxus is carried inwards by the formation of Kolliker's olfactory pit. Thus the 

 dorsal tubercle of the adult subneural gland is not the same thing as the primitive opening of the neuro- 

 hypophysis, but it may be said to correspond with the external orifice of Kolliker's olfactory pit. The 

 olfactory pit and neuropore in Amphioxus together . represent the neuro-hypophysis of the Ascidian larva; the 

 subneural gland of the adult Ascidian which develops from the neuro-hypophysis is not represented in 

 Amphioxus and is, so far as we know, a purely Ascidian structure. 



2 I offered an explanation of the dissociation of the larval mouth from the neuropore in 1891 (A. Willey, 

 "The later larval development of Amphioxus," Quart. Journ. Micro. Sci., Vol. xxxii., 1891), which has met with 

 some favour. 



3 E. Ray Lankester and A. Willey, "The development of the atrial chamber of Amphioxus," Quart. Journ. 

 Micro. Sci., Vol. xxxi., 1890, p. 445. 



