47 



chorclata, and in the Craniates the notochord arises from the dorsal 

 Hp of the blastopore after the gastrula has been formed. Even 

 if it be said to be derived from the mid-dorsal entoderm it is still 

 post-gastrular in origin and position. The so-called notochord 

 of the Enteropneusta arises from the anterior region of the 

 alimentary canal at a late period, after the stomodeum has been 

 formed, and from its position it is impossible to say that it is 

 entodermal in origin, any more than, for example, the very similar 

 subneiu\al gland of the Tunicates can be proved to be ectodermal, 

 at all events, the duct of that structure. Even if some are anxious 

 to believe that the proboscis diverticulum of Balanoglossus is 

 truly entodermal, then it must be derived from the extreme 

 anterior end of the enteron and is therefore gastrular in origin. 

 If it be said to be stomodeal then it has certainly nothing to do 

 with the dorsal lip of the blastopore. 



Undoubtedly with regard to the pharyngeal gill-slits the 

 Enteropneusta indicate affinities with Amphioxus, but in every 

 other respect they are closely associated with the Pterobranchia 

 and with the Echinoderma. There is a wide hiatus developmentally 

 and structurally between the true Chordata and Balanoglossus, 

 but in development and structure the Echinoderma, the Ptero- 

 branchia and the Enteropneusta have much in common. They 

 present a proctodeal blastopore and a hydrocoele, which latter is 

 retained during life. We would be justified in uniting them under 

 such a name say as Hydrocoela. 



For these reasons also it is plain that Phoronis is excluded 

 from this group. The blastopore is stomodeal, and there is no 

 hydrocoele. The anterior cavity of the larva of Phoronis, as has 

 been stated, is occupied by a blastocoele in which blood cells may 

 be met with from the hood to the larval tentacles. 



We have learned a great deal in recent years as to the fate of 

 the blastomeres, but no morphologist appears to have thought ifc 

 worth while pajdng any attention to the directive influence of the 

 blastopore. Yet it is evident that in the invertebrate groups 

 generally, from Coelenterates onwards, it is stomodeal as it is in 

 Phoronis. When an anus is formed the opening takes place by a 

 fresh communication of the enteron with the ectoderm posteriorly 

 to the blastopore. In the Annehds and the Arthropods the mouth 

 and anus are both derived from the blastopore. In the groups 



