224 



GEOLOGICAL BIOLOGY. 



and we have bilateral symmetry. When there is multiplicate 

 division, whether there is symmetry or not, the rays thus 

 formed are called actinimeres, or ray parts. This mode of 

 differentiation is characteristic of the Ccelenterata and Echi- 

 nodermata (omitting from the former branch the sponges), and 

 suggested to Cuvier the name Radiata (see Figs. 1419). 



Somites, Arthromeres, and Diarthromeres of the Arthropods. 

 Another large and diverse group of organisms is character- 

 ized by repetition of parts in the direction of the longitudinal 

 axis. The technical name for body without 

 its parts is soma; the repeated parts which 

 are longitudinally multiplied are called mc- 

 tameres, somites, or segments (see Fig. 55). 



The annelids represent the simple metam- 

 eric type, without appendages to the sepa- 

 rate metameres or segments. In the higher 

 class, the Arthropoda, including the Crustacea, 

 Myriapods, Insects, etc., the so- 

 mites are provided with lateral 

 appendages which are jointed in 

 regular manner (see Fig. 56, also 

 Fig. 50). 



In the Arthropoda, such as 

 the common lobster, and in an 

 insect, these separate somites 

 form a single ring enclosing the 

 interior organs ; but in the Ver- 

 tebrates the somite is double, the 

 FIG. 5 6. A me- special system of correlation ly- 



tameric animal ... . , 



with jointed ing m the upper arch, the organs 



appendages, . 



Scoiofendreiia of assimilation or auxiliary lunc- 



iminaculata. . i 



Leunis.) tion lying in the cavity below. 

 To distinguish these two forms 

 of the metameres the first is called 

 a joint part, arthromere ; the corresponding part in the verte- 

 brate structure is called a two-joint part, diarthromere. The 

 joints of the appendages of a rnetameric part, as the joints of 

 the legs of a lobster or the several bones in the limbs of ver- 

 tebrates, are illustrations of multiplication of parts by division 



type, a diagram of a 

 typical annelid, m = 

 mouth ; ce = cerebral 

 ganglion ; n = ventral 

 nerve : cord ; pr x head 

 (prostomium) ; a = 

 anus. The body 

 (soma) is composed of 

 twenty-five segments 

 or metameres. 



