DE VEL OPMENT. 42 1 



Affinities with Vertebrates (especially emphasised by Mr. Bateson). 



(1) " Notochord. " A dorsal outgrowth from the anterior region 



of the gut grows forward for a short distance into the pro- 

 boscis, and becomes a solid supporting rod (Fig. 207, NchJ], 

 It may be compared with the notochord of Vertebrates, 

 which also arises dorsally from the gut. But it lies below 

 the main dorsal blood vessel, is of very limited extent, and 

 may be merely an analogue of the notochord. 



(2) " Gill-slils." -Numerous gill-slits (Fig. 205) open from the 



anterior region of the gut to the exterior, and are separated 

 from one another by skeletal bars, which in some ways 

 resemble the framework of the respiratory pharynx in 

 Amphioxus. There are, however, many differences in 

 detail, thus the slits open dorsally, not laterally ; the 

 skeletal bars are differently disposed ; the blood supply is 

 different. 



(3) '''Dorsal nerve-cord.' 1 '' A dorsal median insinking (Fig. 206, 



d.n.} of ectoderm, especially strong in the region of the 

 collar, may be compared with the medullary canal of Verte- 

 brates. But it must be noticed that there is also a ventral 

 nerve-cord (Fig. 206, v.n.}. 



Mr. Bateson has also noted that the mesoblast arises, as in Amphioxus, 

 etc. , in the form of ccelom pouches, but this is true of many Inverte- 

 brates. He states that the history of the anterior ccelom pocket, which 

 grows forward in the proboscis of Balanoglossus^ is closely like that in 

 Aniphioxns, but this is denied by Spengel. He compares a slight fold, 

 which in some species grows backwards from in front of the gill-slits, 

 with the epipleural folds of Amphioxus (Fig. 205, op.}. The numerous 

 serially arranged gonads recall those vi Amphioxus. It is still uncertain 

 what weight should be attached to the fact that Balanoglossus is un- 

 segmented. 



Affinities with Annelids (after Professor Spengel). 



(1) The larva (Tornaria) (Fig. 208) may be regarded as a modified 



Trochosphere, but this points at most to a far-off common 

 stock. Moreover, the nephridia, usually present in the 

 Trochosphere, are unrepresented in the Tornaria. 



(2) The body cavity is formed from segmentally arranged ccelom 



pouches ; but there is a pair of pre-oral pouches absent in 

 Annelids, and the segmental arrangement in the organs of 

 the body in Balanoglossus is. to say the least, very vague. 



(3) The heart lies, as in some Annelids, dorsal to the gut, not 



ventral as in Vertebrates ; the dorsal vessel carries blood 

 forwards, the ventral backwards, as is usual in Annelids. 

 But the double nervous system is essentially different from 

 that of Annelids ; and the gill-slits are also, so far as we 

 know, unrepresented there. If there be a relationship 

 between Enteropneusta and Annelids, it must be a very 

 distant one, perhaps restricted to origin from some common 

 stock. 



