ANCESTRY OF CHORDATES. 575 



mouth-plate. The latter contains a short notochordal rod ; and there is a single 

 pair of gill-slits opening from the pharynx, water being passed into this from the 

 mouth by the action of the tentacles. In the allied genus Rkabdoplewa the 

 individuals which go to form a colony are connected with one another by means 

 of a common stem, representing the remnants of their original contractile stalks ; 

 this stem gradually drying up with the growth of the colony in the region most 

 remote from the living polyps. Each polyp has but a single plume-like tentacle ; 

 and the buds arising from the soft part of the common stem never become 

 detached. While the nervous system and notochord are essentially the same as in 

 Cephalodiscus, gill-slits are wanting. 



Ancestry of Before making a few brief remarks on this interesting but 



Chordates. perplexing subject, it may be mentioned that while we have no 

 satisfactory clue as to the first origin of the notochord, it has been suggested that 

 the original function of gill-slits was to carry off the superfluous water entering 

 the mouth with the food ; the connection with respiration being a later addition 

 to these structures. It is also an important factor in the consideration of this 

 subject to bear in mind that the whole of the existing Protochordates are to a 

 greater or less extent degenerate types, although they doubtless retain some original 

 and simple primitive features. For the proud position of the original ancestral 

 stock, from which have sprung both Protochordates and Vertebrates, there are 

 many claimants ; among these being segmented worms or annelids, creatures allied 

 to the existing king-crab, and star-fishes and sea-urchins. With regard to the 

 annelid theory, Mr. Willing very significantly remarks that in this case the doctrine 

 of parallelism in development has not been sufficiently taken into account ; and 

 that the more complete the superficial resemblance between an Annelid and a 

 Vertebrate, in the same measure is the parallelism in their developmental history 

 the more striking, and their genetic affinity the more remote. Neither is it likely 

 that the king-crab line of descent (in spite of the apparent identity in the structure 

 of one layer of its shell with that of the Cephalaspidians) will hold good. The 

 evidence in favour of an alliance between Vertebrates and Echinoderms (sea-urchins 

 and star-fishes), through the intervention of Balanoglossus, seems, however, to be 

 steadily gaining ground. Mr. Willey, for instance, remarks that while it is 

 probable that the proximate ancestor of the Vertebrates was a free-swimming 

 creature, intermediate in structure between an ascidian larva and the lancelet, the 

 ultimate or primordial ancestor may be assumed to have been a worm-like animal, 

 with an organisation approximately on a level with that of the bilaterally sym- 

 metrical progenitors of the Echinoderms. Mr. Garstang also, having proved that 

 the larvae of the whole of the latter group can be derived from a single common 

 type, and likewise having shown that the tornaria-larva of Balanoglossus can be 

 referred to the same modification, expressed an opinion that the Vertebrates also 

 trace their origin to the same free-swimming pelagic form. Perhaps still more 

 probability may attach to a later theory of the same observer, who now comes to 

 the conclusion that Echinoderms, Enteropneusta, and Chordates are all divergent 

 branches from a common unknown ancestor; such ancestor being a bilaterally 

 symmetrical creature with the general appearance of a certain type (Auricularia) 

 of Echinoderm larva. From the hypothetical common stock the Echinoderms 



