PR&ORAL LOBE. 277 



praeoral lobe. The origin of the main cavity of the prae- 

 oral lobe in Amphioxus from the right of a symmetrical 

 pair of head-cavities (anterior intestinal diverticula of 

 Hatschek) has been described in a previous chapter. In 

 Balanoglossus there is no such complete division of the 

 praeoral body-cavity, but it is throughout a single space, 

 its right and left halves being confluent. If we now com- 

 pare the condition of things in the embryo of Amphioxus, 

 where we have a symmetrical pair of head-cavities, with 

 that of some other form which, in the adult condition, 

 possesses a distinct pair of such cavities, it may assist us 

 in imagining how the mouth could have assumed such 

 opposite relations as have been mentioned above. 



But first it may be pointed out that in Appendicularia, 

 where, as it would appear, in correlation with the second- 

 ary acquirement of a purely pelagic habit of life (although 

 this point of view is not shared by such authorities as 

 Herdman, Seeliger, and Brooks), the praeoral lobe has 

 been reduced to a minimum, or to zero, the mouth has 

 thereby come to lie in a terminal, or sub-terminal, position, 

 with a slight tendency towards the dorsal side.* 



In the curious pelagic worm, Sagitfa, we meet with 

 another instance of an animal in which the praeoral lobe, 

 in the ordinary sense of the term, is reduced to a mini- 

 mum, and the mouth has therefore a sub-terminal position, 

 with a ventral inclination (Fig. 132). But although there 

 is no distinct praeoral lobe in Sagitta, there is, neverthe- 

 less, a pair of head-cavities, which are directly comparable, 

 if not perfectly homologous, with the above-mentioned 



* Whatever the truth may be as to the precise systematic position and 

 phylogenetic value of Appendicularia, one thing, to my mind, remains abso- 

 lutely certain, namely, that it has descended from a form which possessed a 

 praeoral lobe, and that it has secondarily lost that structure. 



