xni GENUS AND SPECIES 139 



bears at its distal end about six zooids in a cluster. The 

 zooids are more elongated than in any of the preceding 

 species, and there are no special reproductive individuals, so 

 that the colony is homomorphic. 



In Z. affine (Fig. 27, D) the stalk is dichotomous but is 

 proportionally thicker than in the preceding species, and 

 bears about four zooids, all alike. It is found in fresh water 

 attached to insects and other aquatic animals. 



The last species we shall consider is Z. nutans (Fig. 27, E), 

 which is the simplest known, never bearing more than two 

 zooids, and sometimes only one. 



A glance at Figs. 26 and 27 will show that these six species 

 agree with one another in the general form of the zooids, in 

 the characters of the nucleus, contractile vacuole, &c., in 

 the arrangement of the cilia, and in the fact that they are all 

 compound organisms, consisting of two or more zooids 

 attached to a common stem, the axial fibre of which branches 

 with it, i.e., is continuous throughout the colony. 



On account of their possessing these important characters 

 in common, the species described are placed in the single 

 genus Zoothamnium, and the characters summarized in the 

 preceding paragraph are called generic characters. On the 

 other hand the points of difference between the various 

 species, such as the forking of the stem in Z. dichotomum, 

 the presence of only two zooids in Z. nutans, and so on, are 

 called specific characters. Similarly the name Zoothamnium, 

 which is common to all the species, is the generic name, 

 while those which are applied only to a particular species, 

 such as arbuscula, simplex, &c., are the specific names. As 

 was mentioned in the first lesson (p. 8), this method of 

 naming organisms is known as the Linnean system of 

 binomial nomenclature. 



It will be seen from the foregoing account that by a 



