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 Rhabdopleura 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 theorj 7 , 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 Ecliinoderms (sea-urchins 
and star-fishes), through the intervention of Balcmoglossus, 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 Ecliinoderms. 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 Balanoglossns 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 Ecliinoderms, 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 ( Auricidaria ) 
of Echinoderm larva. From the hypothetical common stock the Ecliinoderms 
