578 ZOOLOGY SECT. 



The propriety of including the Adelochorda 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 improbable 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 

 Adelochorda 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 neurocoele of 

 Urochorda and Vertebrata, which extends, from the first, through 

 the whole length of a well-defined dorsal nervous system. In 

 Cephalodiscus and Rhabdopleura, moreover, there is no trace of 

 any such cavity. 



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

 different thing from the notochord of Urochorda and Vertebrata, 

 which is formed as a rod separated from the entire mid-dorsal 

 region of the mesenteron, nothing in its structure or development 

 giving the slightest indication that it originally arose as a forward 

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

 culum of Adelochorda 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 orgin. 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 

 Actinotrocha, and in Cephalodiscus are a single pair of apertures, 

 having apparently no respiratory function. In Balanoglossus, 

 however, they are very numerous and increase in number with the 



rwth of the animal, as in Amphioxus, and the division of each 

 ^ a " tongue " is very similar in the two cases. Further homologies 

 have been suggested by comparing the snout of Amphioxus with 

 the proboscis or pre-oral lobe of Adelochorda and its pre-oral pit 

 with the proboscis-pore. 



On the whole, although it is by no means certain that the 

 " chordate " peculiarities of the Adelochorda may not have been 

 independently evolved, it is convenient to retain them in the 

 present phylum, pending further knowledge of their true affinities. 

 By various zoologists the Chordata have been supposed to be 

 derived from Nemertinea, from ChaBtopoda, and from Arthropoda. 

 In the Nemertinea the proboscis sheath has been compared with 

 the notochord and the proboscis itself with the pituitary invagina- 



