CHAETOGNATHA. 591 



and divides into an anterior and posterior part on each side. The 

 former is said to constitute the ovary, and the latter the testis. 



As stated above the diaetognatlia are essentially pelagic organisms. 

 They swim by flexion of the body, and have been taken at 

 considerable depths (600 fathoms). One species is littoral (Spadella 

 cephaloptera). 



The Chaetognatha, in possessing three divisions of the coelom, resemble the 

 Brachiopoda, on the assumption that the gastro- and ilioparietal bands of the 

 latter group are the remains of transverse coelomic septa (p. 580). In the 

 same feature they also resemble Balanoglossus. The resemblance to the latter 

 is, however, diminished by the fact that the division of the coelom into three 

 chambers is not an original one, but occurs subsequently to the separation from 

 the archenteron. There is this further difference, viz., in Balanoglossus it is the 

 anterior portion of the embryonic coelom which is unpaired and median, while 

 in Chaetognatha it is the posterior part. With regard to the latter point it is 

 possible that further investigations of the embryos of the Chaetognatha with 

 modern methods may disclose some errors in the old accounts, and with regard 

 to the first, it must not be forgotten that even in the same group the coelomic 

 sacs may differ as to when they divide up transversely; e.g., in the Echino- 

 dermata in which group it is probable that the typical arrangement of coelomic 

 sacs is one unpaired and two paired the sacs may in some forms arise separately 

 from the archenteron, and in others arise by the division of one archenteric 

 diverticulum on each side. 



Three genera of Chaetognatha are known. 



SarjMa Slabber, with two pairs of lateral fins. 



Krohnia Langerhans, with one lateral fin on each side extending on to the 

 caudal region. 



Spadella Langerhans, with one pair of lateral fins on the tail, and a thicken- 

 ing of epidermis on each side of the body from the head to the fin. 



END OF VOL. I. 



