DE VEL OPMENT. 393 



proboscis and communicates with the exterior, has also been compared 

 with the beginning of the water vascular system in Echinoderms, and it 

 is true that in both several independent coelom pouches grow out from 

 the primitive gut. The anterior body cavity in Balanoglossus com- 

 municates with the exterior by a pore, which becomes the proboscis-pore 

 of the adult, and this has been compared with the water-pore, or outlet 

 of the water vascular system of Echinoderms, which similarly opens 

 from an anterior enterocoel to the exterior. On the other hand, the 

 presence of an apical plate — a structure almost invariably absent in 

 Echinoderms, suggests an affinity with an Annelid trochosphere. 



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. 171, Nch.). 

 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 veiy limited extent, 

 and may be merely an analogue of the notochord — a 

 physiological necessity for the support of the elongated 

 proboscis. 



(2) "Gill-slits." — Numerous gill-slits (Fig. 169) 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. Nor is it certain that the gullet of Balanoglossus 

 is endodermic like that of Vertebrates. Still, the possession 

 of these respiratory slits is one of the most satisfactory of the 

 alleged Vertebrate-like characters of Balanoglossus. 



(3) " Dorsal nerve-cord." — A dorsal median insinking (Fig. 170, 



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. 170, 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 

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

 which grows backwards from in front of the gill-slits, with the epipleural 

 folds (A Amphioxus (Fig. 169, op.), but the fold appears to be restricted 

 to one species. It is still uncertain what weight should be attached to 

 the fact that Balanoglossus is unsegmented. 



Affinities with Annelids (after Prof. Spengel). 



(1) The larva (Tornaiia) (Fig. 172) 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. 



