XIII 



PHYLUM CHORDATA 503 



putting them aside as of no phylogenetic importance, it is obvious 

 that this group must have sprung from a point very low down the 

 chordate stem. The morphological differences between Amphioxus 

 and a Hag are, in fact, of a more fundamental character than 

 those between a Hag and a Mammal. 



Still lower must have been the point of origin of the Urochorda, 

 with the notochord confined to the tail, the dorsal mouth, the 

 absence of myomeres and of nephridia, and with only exceptional 

 and ill-defined traces of segmentation. The huge pharynx with 

 its innumerable stigmata is undoubtedly a secondary character, ; 

 but the atrium, endostyle, dorsal lamina, and peripharyngeal bands 

 seem to indicate an affinity with the Acrania. So also do the 

 earlier stages of development ; but the later stages, and especially 

 the mode of origin of the atrium, are quite different in the 

 two cases. 



The propriety of including the Hemichorda among the Chordata 

 is still sub judice. Allowing that any single organ may have a 

 polyphyletic origin i.e., may arise independently in different 

 groups in accordance with similar needs it seems highly impro- 

 bable that three such peculiar and characteristic structures as 

 notochord, hollow dorsal nervous system, and gill-slits can have 

 arisen together more than once in the history of animals ; and if it 

 could be shown with certainty that these three characters were all 

 present in the Hemichorda their place in the chordate phylum 

 would be assured. But the cavity or cavities in the dorsal nerve- 

 cord of Balanoglossus are inconstant, and are very different from 

 the neuroccele of Urochorda and Vertebrata, which from the first 

 extends through the whole length of a well-defined dorsal nervous 

 system. In Cephalodiscus and Khabdopleura, moreover, there is 

 no trace of any such cavity. 



The pharyngeal diverticulum of the Hemichorda, also, is a very 

 different thing from the notochord of Urochorda and Vertebrata, 

 nothing in the structure or development of which gives the 

 slightest indication that it originally arose as a forward out- 

 growth of the anterior portion of the mid-gut. The diverti- 

 culum of Hemichorda is, in fact, obviously a support to the per- 

 sistent prostomium of a fixed or sluggish animal, while that of 

 Urochorda and Vertebrata forms a strengthening axis either to 

 the tail alone or to the whole body of an active, elongated, 

 animal swimming by lateral movements of the tail ; and there 

 seems to be no reason why two such different structures should 

 not have had an independent origin. The supposed double 

 " notochord " of Actinotrocha, the larva of Phoronis, is even more 

 problematical. 



Far more significant are the gill-slits, but even their evidence is 

 hardly conclusive, since they are absent in Rhabdopleura and 



