270 GILL-CLEFTS. 



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 characteristic organs 

 of the Chorclata, 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 Balauo- 

 glossus 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 arill-clefts. 



o o 



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

 lesophageal diverticula, which are eventually placed in communication with 

 the exterior by a pair of ciliated canals (vide Vol. i. pp. 164 and 166), are 

 such organs. 



Semper (No. 256) lias made the interesting discovery that in the buckling 

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

 lumen may be formed, unite with the oral invagination and primitive alimen- 

 tary 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 011 this subject 

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

 interpretation. 



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" to take place in the fore part 

 of the alimentary tract of many Cluetopods. This part of the alimentary 

 tract was probably provided with paired ca^cal pouches with their blind 

 ends in contiguity with the skin. 



Perforations placing these pouches in communication with the exterior 

 must be supposed to have been formed; and the existence of openings into 

 the alimentary tract at the end of the tentacles of many Actinia? and of the 

 hepatic diverticula of some nuclibranchiate Molluscs (Eolis, ifec. 3 ) shews that 

 such perforations may easily be made. On the formation of such per- 

 forations the water taken in at the mouth would pass out by them ; and 

 the respiration would be localized in the walls of the pouches leading to 



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

 notochord, but none of these interpretations will bear examination. 



- " Ueb. d. Vorkommeu eines schwimmblasenahnlichen Organs bei Anneh'den." 

 llittlieil. u. d. zool. Station ;n X cupel, Vol. n. 1881. 



3 The openings of the hepatic diverticula through the sacks line;! w ith thread cells 

 are described by Hancock and Embletou, Ann. and Mmj. of Xnt. History, Vol. xv. 

 1845, p. 82. Von Jhering has also recently described these openings (Zool. Anzciijcr, 

 No. 23) and apparently attributes their discovery to himself. 



