LATER LAEVAL DEVELOPMENT OF AMPHIOXQS. 211 



special outlet for any nauseous substances entering the mouth, 

 which might otherwise act injuriously on the endostyle. But 

 the histological structure of the cells and the physiological 

 properties of the gland require further study before any safe 

 conclusion can be arrived at as to its function. The cells of 

 the gland are ciliated, and the external orifice is surrounded by 

 cilia. 



4. The Mouth or Velum. 



The mouth arises in an embryo of about thirty-two hours as 

 a minute circular aperture on the left side in the region of the 

 first myotome (see Hatschek, loc. cit.). It then grows rapidly 

 in size, and becomes a large lateral lens-shaped orifice, more 

 or less pointed at both ends. From Stage I onwards it begins 

 to grow gradually round the anterior extremity of the alimentary 

 canal, the change in position being accompanied by great alte- 

 ration in shape, consisting chiefly in the gradual equalisation 

 of its diameters, which are at first remarkably unequal, the 

 longer diameter being four or five times the length of the 

 shorter one. The bending round of the mouth commences by 

 a curious hunching up of the antero-dorsal margin of the mouth 

 into the ciliated groove which runs from the prseoral pit to the 

 dorsal edge of the mouth. 



By this means the previously pointed anterior extremity of 

 the mouth becomes lost, and this part of the mouth then pre- 

 sents a truncated appearance. Meanwhile the anterior half of 

 the mouth begins to bend inwards towards the right side, and, 

 in fact, it eventually becomes the right half of the velum. The 

 gradual attainment by the mouth of an anterior median and 

 vertical position is fully illustrated in the plates at the end of 

 this paper. The mouth is relatively much smaller in the adult 

 than in the larva, but not actually so. 



The edge of the larval mouth is provided with two kinds of 

 cilia, namely, (i) a set of evenly distributed small cilia, and (ii) 

 at a rather deeper focus much longer cilia, which hang together 

 in groups at equal intervals. At the anterior extremity of the 

 mouth in the earlier stages there is a group of very long cilia, 



