424 ZOOLOGY. 



of junctioii. Finally, at other times, their union is -till 

 more complete, and it is only by analogy that lln- /oologist 

 is induced to consider the segment resulting from their fusion 

 as composed of several rings rather than one. There results, 

 as might be expected, very wide differences in the general 

 form of these animals ; and if we compare together a cloporte 

 (Fig. 419) or wood louse, a talitrus (Fig. 146),* and a crab 

 (Fig. 420), for example, one might be led at first sight to 

 believe them to be constructed on wholly different types ; but 

 a deeper study of their structure shows that the composition 

 of their tegumentary skeleton is essentially the same, and that 

 the differences depend on this, that the greater number of the 

 rings, completely distinct and moveable in the oniscus, are 

 united to each other in the crabs, and that certain analogous 

 parts do not present in these two genera the same proportions. 

 Thus, in the oniscus (Fig. 419) 

 or in the talitrus (Fig. 146) we 

 find a distinct head (c), followed 

 ky a thorax composed of seven 

 rin S s > resembling each other (t\ t"), 

 and carrying each a pair of limbs 

 (p,pp}i finally, at the posterior 

 part of the body we find an ab- 

 domen (ab), formed also of seven 

 segments, whose size diminishes 

 rapidly, but whose form is nearly 

 the same as in the thorax. In a 

 crvfo, on the contrary (Fig. 420), 

 Fig. 419. Cloporte (Oniscus). the head is not distinct from the 

 thorax, and forms with all this 



section of the body a single segment covered with a large solid 

 buckler, named shell or carapace; finally, the abdomen at first 

 escapes the sight, because it is folded under the thorax, and is 

 but small. Nevertheless, it is easy to demonstrate that in the 

 crab, as in the oniscus, there exist behind the head seven easily 

 recognisable thoracic rings, and that the carapace is not a new 

 organ created for the former, but merely the dorsal portion of one 

 of the rings of the head which has acquired an extreme develop- 

 ment, and encroached on the neighbouring rings. In other 

 animals of the same class, the general form of the body 

 is removed still further from those of which we have just 



Oniscua locustraj talitrus: Lat. Of the genus Gammarina ; family, 

 Oammarus. B. K. 



