326 GILL-CLEFTS. 



and no invertebrate group can fairly be regarded as genetically 

 related to the Chordata till it can be shewn to possess some 

 organ either derived from a notochord, or capable of having 

 become developed into a notochord. No such organ has as yet 

 been recognized in any invertebrate group 1 . 



Gill-clefts. The gill-clefts, which are essentially pouches of 

 the throat opening externally, constitute extremely character- 

 istic organs of the Chordata, and have always been taken into 

 consideration in any comparison between the Chordata and the 

 Invertebrata. 



Amongst the Invertebrata organs of undoubtedly the same 

 nature are, so far as I know, only found in Balanoglossus, where 

 they were discovered by Kowalevsky. The resemblance in this 

 case is very striking ; but although it is quite possible that the 

 gill-clefts in Balanoglossus are genetically connected with those 

 of the Chordata, yet the organization of Balanoglossus is as a 

 whole so different from that of the Chordata that no comparison 

 can be instituted between the two groups in the present state of 

 our knowledge. 



Other organs of the Invertebrata have some resemblance to the gill-clefts. 

 The lateral pits of the Nemertines, which appear to grow out as a pair of 

 oesophageal diverticula, which are eventually placed in communication with 

 the exterior by a pair of ciliated canals (vide Vol. II. pp. 200 and 202), are 

 such organs. 



Semper (No. 256) has made the interesting discovery that in the budding 

 of Nais and Chaetogaster two lateral masses of cells, in each of which a lumen 

 may be formed, unite with the oral invagination and primitive alimentary 

 canal to form the permanent cephalic gut. The lateral masses of cells are 

 regarded by him as branchial passages homologous in some way with those 

 in the Chordata. The somewhat scanty observations on this subject which 

 he has recorded do not appear to me to lend much support to this interpre- 

 tation. 



It is probable that the part of the alimentary tract in which gill- clefts 

 are present was originally a simple unperforated tube provided with highly 

 vascular walls ; and that respiration was carried on in it by the alternate 

 introduction and expulsion of sea water. A more or less similar mode of 

 respiration has been recently shewn by Eisig 2 to take place in the fore part 



1 In the Chaetopods various organs have been interpreted as rudiments of a 

 notochord, but none of these interpretations will bear examination. 



2 " Ueb. d. Vorkommen eines schwimmblasenahnlichen Organs bei Anneliden." 

 Mittheil. a. d. zoo!. Station zu Neapel, Vol. n. 1881. 



