STRUCTURE OF BALANOGLOSSUS. 351 



Before we give a connected account of the structure of 

 JBalanoglossus, its striking resemblances with Chordata, and 

 especially with the lancelet-type, may be summarised from 

 Bateson's memoir on the subject. 



(i) The dorsal nen'ous system arises from the skin along 

 the mid line of the back. . 



(2) The notochord arises as a supporting structure in the 

 anterior end of the body by the constriction of 

 a portion of endoderm from the gut. 



(3) The gill-slits are formed as regular fusions and per- 



forations of the body-wall and gut from before 

 backwards. " Hence the three features which alone 

 distinguish Chordata from other animals are present, 

 and associated from an early period in development." 

 Added to these, some minor features of Chordate 

 anatomy are also represented by :■ — 



(4) The origin of the mesoblast, from pouches of the 



primitive gut : 



(5) The remarkable asymmetry of the anterior parts, e.g., 



an anterior pouch cut off from the primitive gut, and 

 opening externally, is paralleled in some measure in 

 Amphioxus : 



(6) The slight opercular fold, a growth of the " collar " 



over the gill-slits, perhaps comparable to the folds in 

 Amphioxus : 



(7) The excretory funnels opening into the slight atrial 



cavity thus formed. 

 On the other hand, it should be recognised, (a) that in 

 addition to the dorsal nerve-cord, there is in Balanoglossus 

 a ventral nerve-cord, a band round the pharynx, and a plexus 

 beneath the skin ; (b) that the " notochord " ol Balanoglossus 

 lies ventral, not dorsal, to the main dorsal blood-vessel, and 

 may be only an analogue, not a homologue, of the notochord 

 of higher animals ; (c) that Balanoglossus is an unsegmented 

 organism, whereas the higher forms are segmented, but it is 

 possible that this diiference is rather of physiological than of 

 morphological importance. In short, the student must 

 recognise that there is little certainty of knowledge in regard 

 to the more detailed problems of organic relationship. Even 

 the well-weighed opinions of the experts are to be accepted, 

 as they are in most cases offered, as probabilities. 



