EVIDENCE FROM COLONIAL OR COMPOUND ANIMALS. 285 



to eggs. Each egg, undergoing its full development, produces first 

 one primitive unit, and thereafter and from this unit develops, by 

 budding, a whole colony, with its hundreds of component and similar 

 beings. 



There exist in the ranks of that curious class of beings, the in- 

 ternal parasites, certain interesting examples of the compound animal 

 form. A tapeworm (Fig. 191), for instance, inhabiting the digestive 

 tract of some warm-blooded quadrupeds, and attaining a length, it 

 may be, of many yards, consists of a very minute head (Fig. 191, i), 

 a slender neck, and many hundreds of so-called "joints." At first 

 sight, these "joints" might be re- 

 garded as resembling in their nature 

 those of the ordinary worms, and 

 as therefore possessing no distinct 

 individuality on their own account, 

 or separate ^rom that of the organism 

 of which they form part. But the 

 examination of the joint of a tape- 

 worm (Fig. 191, 2) shows us that in 

 reality it preserves a separate and 

 apparent individuality of its own. In 

 other words, it is not merely a part of 

 one animal in the sense that the joint 

 of a backbone is part of a fish or bird. 

 It corresponds, on the contrary, with 

 a member of the zoophyte or " sea- 

 mat" colony in that it represents a 

 highly specialised and individualised 

 unit of an organism, that organism 

 being of compound nature. Each 

 "joint" of the tapeworm contains a 



complete set of egg-producing organs (0), and presents other indica- 

 tions of its semi-independent character and constitution. Connected 

 to its neighbouring joints by water-vessels as well as by the nerve- 

 cords, the joint is in intimate union with the other units of the colony. 

 But it is, nevertheless, a distinct unit after all ; and the tapeworm is 

 not a single animal, but, like the sea-mat and zoophyte, a " colonial 

 organism." 



Amongst other and true "worms," however, we find curious 

 instances of development, which, in our consideration of the 

 origin of the conditions we are studying, may serve to elicit some 

 valuable hints concerning the causation of colonies at large. The 

 little river-worms known as the Nai'dides (Fig. 192), occasionally ex- 

 emplify certain peculiar modes of reproduction which deserve careful 

 study. A nais may be seen to exhibit a slight constriction towards the 



FIG. 191. TAPEWORM. 

 Head, suckers, neck, and joints ; 

 2. A single joint (magnified). 



