PR.^ORAL LOBE. 



-77 



prajoral lobe. The origin of the main cavity of the pra;- 

 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 

 prseoral 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 ha\"e 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 mav 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 Appcndicularia, 

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

 ar\- 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 prjeoral 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, Sagitta. we meet with 

 another instance of an animal in which the prjeoral 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 prasoral lobe in Sagitta, there is, neverthe- 

 less, a /(?/;- c'_f//d-(7(/-i-iK7V/Vj-, which are directlv comparable, 

 if not perfectly homologous, with the above-mentioned 



* ^^^latever 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 secondarilv lost that structure. 



