CHAETOGNATHA 



545 



with its glassy transparent body and its powers of vertical migration ; 

 off the coast of California it lives at a depth of 15-20 fathoms during 

 the day and the greater part of the night, but at sunrise and sunset it 

 rises to the surface, the light intensity and temperature there being 

 at an optimum for the species at those times. 



The Chaetognatha are a very early offshoot of the coelomate stock 

 and cannot very well be compared to any other phylum. While it is 

 tempting to liken the tripartite division of the coelom in Chaeto- 

 gnatha with that in echinoderms and protochordates, it must be 

 realized that in Sagitta the two transverse septa arise at different 



Fig. 405. Larvae of Sagitta showing formation of coelomic pouches from the 

 archenteron . After Burfield . In A the pouches still open into the archenteron . 

 In B the pouches forming the head coelom have completely separated off from 

 the archenteron and the archenteric folds have grown back so as partly to 

 separate off the second pair of pouches, al.c. alimentary canal ; his. blastopore ; 

 ect. ectoderm ; end. endoderm ; g.c. genital cells ; hd.coe. head coelom ; M. 

 mouth; std. stomodaeum. 



times and for different reasons. (There is, however, a true tail here 

 which is elsewhere found only in the Chordata.) 



The fossil, Amiskwiay occurring in the Cambrian, has been assigned 

 to this group, but it appears to differ from the living forms in the 

 absence of a septum between trunk and tail and in the presence of 

 tentacles on the head. 



PHYLUM PHORONIDEA 



Coelomate unsegmented animals, sedentary, hermaphrodite and tubi- 

 colous, with a horseshoe-shaped lophophore, an epistome, a vascular 

 system with haemoglobin, and two segmental excretory organs. 



This is a very small group : the genus Phoronis (Fig. 406) includes 

 most of the species. They are all marine animals, usually of incon- 

 siderable size, and like all sedentary forms they have a free-swimming 

 larva; this is called an Actinotrocha and it can be referred to the 



Bi . 35 



