622 THE INVERTEBRATA 



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

 rises to the surface, the Hght 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 

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

 which is elsewhere found only in the Chordata and the development 

 of the body cavity is enterocoelic.) 



The fossil, Amtskwia, 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 excretory organs. 



This is a very small group: the genus Phoronis (Fig. 431) 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 

 trochosphere type. It passes into the adult by a remarkable meta- 

 morphosis which is illustrated in Fig. 432. 



Phoronis has a strong resemblance to a polyzoan like Plumatella but 

 it differs from such a form in the presence of a vascular system and 

 in other respects. 



