CHAPTER XIV 

 CHAETOGNATHA 



THE little group of the Chaetognatha, or " Arrow-worms," cousistiug 

 of Sagitta, Spadella, and Jfrohnia, is one of the most isolated in the 

 animal kingdom, and its affinities are extremely difficult to determine. 

 All we can say with certainty is that its members exhibit a very 

 primitive type of adult structure and a still more primitive develop- 

 ment. 



SAGITTA 



The Chaetognatha are with few exceptions pelagic animals and 

 are of world -wide distribution. The development of the genus 

 Sagitta has been worked at by Kowalevsky (1871), Btitschli (1873), 

 and by Hertwig (1880), who described what could be made out from 

 observations on the living embryos and from preserved specimens 

 mounted whole. The subject has been taken up again by Doncaster 

 (1902), who used the method of sections and who has given us a fairly 

 complete account of the development of Sagitta, lipunctata. It must 

 be admitted, however, that some of Doncaster's figures are far from 

 satisfactory, and it is greatly to be desired that a revision of the 

 subject should be made. 



Doncaster's method of procuring the eggs and embryos was, 

 to secure a number of adults and keep them in a jar of clean sea- 

 water, in which they lived very well for a day or two, and then to 

 siphon off the bottom layer of the water and to look for the eggs in it. 

 Doncaster preserved the eggs and larvae in a concentrated solution 

 of corrosive sublimate to which 5 per cent of acetic acid was added. 

 He used the method of double embedding in celloidin and paraffin, 

 so frequently recommended in this volume and described in Chapter II. 



Practically the whole development of Sagitta is completed within 

 the egg : membrane, and when the embryo emerges and becomes a 

 larva it differs from the adult chiefly in size and in the absence of 

 developed genital organs. The whole development up to hatching- 

 only occupies about two days. 



The egg, which is '2 mm. in diameter, segments perfectly regularly 

 into blastomeres of approximately equal size. A blastocoele 



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