36 



ANIMAL FORMS 



essentially the same structure but with various modes of 

 branching (for some of the commoner modes, see Fig. 17)- 

 In some of the higher forms a division of labor has arisen 

 among various members of the association which has led to 

 most interesting results. For example, Fig. 19 represents 

 a species of hydroid found investing the shells of sea-snails 

 occupied by hermit crabs (Fig. 60). To the unaided eye 

 its appearance is that of a delicate vegetable growth, but 

 when placed under the microscope it is found to consist of 



a multitude of Hydra-like 

 animals united by a hollow 

 branching root system con- 

 necting the gastric cavities 

 of all of them (Fig. 19). 

 Certain individuals (a) 

 with tentacles and a mouth 

 resemble a Hydra ; others, 

 without a mouth and ten- 

 tacles, are reduced to a 

 club-like form (b) liberally 

 supplied with nettle-cells 

 upon their free extremi- 

 ties; while the third type 

 (c), likewise devoid of a mouth, possesses rudiments of ten- 

 tacles below which are borne numerous clumps of repro- 

 ductive cells. The first type, the only one possessing a 

 mouth, captures the food, and after digesting it distributes 

 the greater portion to the remaining members by means of 

 the connecting root system ; those of the second form, de- 

 fending the others by means of their nettle-cells against 

 the inroads of a foreign enemy, are the soldiers of the colo- 

 ny; while the third type produces the eggs from which 

 new individuals develop. 



In some of the higher Hydrozoa, the Portuguese man- 

 of-war (Fig. 20), this division of labor has reached a more" 

 advanced stage of development, and in addition the entire 



FIG. 19. An enlarged portion of a hydroid 

 colony (Hydractinia), showing (a) the 

 nutritive polyp, (5) the defensive polyp, 

 and (c) the reproductive polyp. 



