iv] THE SECOND GRADE 107 



possible that division of labour should set in at the 

 very beginning, and that no such thing as a colony, 

 using the word in its usual sense as a number of 

 equivalent individuals all derived from a single parent 

 and still connected together, should ever have existed. 

 The best examples of animals with such a history 

 are the Catenata, a small group, all parasites of certain 

 marine worms, discovered by Dogiel (6) only four 

 years ago, and containing but* one known genus, 

 Haplozoon. The structure of the most primitive 

 member of the group is simplicity itself (Fig. 9, e). 

 It is a single row of cells, one end fixed to the wall of 

 the worm's gut, the other sticking out into the gut- 

 cavity. The cells, however, are by no means similar 

 among themselves. The first one takes over all the 

 business of attachment, and most of the nutrition. 

 Actively movable, it possesses at its anterior end a 

 piercing spine and a bundle of delicate protoplasmic 

 -threads or pseudopodia, which insinuate themselves far 

 up between the cells lining the host's digestive tube, 

 and serve the double purpose of holding the parasite 

 firm and of sucking up the juices of the neighbouring 

 tissue. From its posterior end this head-cell is con- 

 tinually dividing off new cells, which remain attached 

 to each other in series, up to some seven or eight. 

 The hinder cells of the series gradually become filled 

 with particles of reserve food, analogous to the yolk 

 granules in an egg, and finally lose their connection 



