THE WORM-LIKE PROTOCHORDATES 2953 



fected by means of this pedicle and the 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 essen- 

 tially the same as in Cephalodiscus, gill slits are wanting. 



Before making a few brief remarks on this interesting but perplex- 

 Ancestry of 



Chordates * n & 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 sim- 

 ple 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 starfishes and sea urchins. With regard to the annelid theory, Mr. 

 Willey 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 favor of an alli- 

 ance between Vertebrates and Echinoderms (sea urchins and starfishes), 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 organization approximately on a level 

 with that of the bilaterally symmetrical progenitors of the Echinoderms. Mr. Gar- 

 stang also, having proved that the larvae of the whole of the latter group can be de- 

 rived 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 



