CHAPTER VII 



BACKBONELESS ANIMALS (INVERTEBRATA). STRUCTURE 

 AND CLASSIFICATION OF NEMERTINES AND MOLLUSCS 



A brief account of the Backboned or Vertebrate animals has 

 now been given, and in accordance with the usual custom from 

 the time of Aristotle downwards all the remaining forms, far 

 more numerous than they, may be conveniently lumped together 

 as Backboneless animals or Invertebrates, divisible into a number 

 of great groups or phyla, each of which is on a footing with the 

 phylum Vertebrata. The lower Invertebrates are so unlike the 

 Vertebrates that close comparison is not possible, but there are 

 certain features which broadly serve to mark off a higher In- 

 vertebrate from a typical backboned animal. These are, to a 

 large extent, implied in the summary given previously (pp. 60—63) 

 of the chief Vertebrate characters, but it may be useful at this 

 point to take such a form as a Cray-Fish or Lobster and point 

 out the distinctive features in question (fig. 176). 



The body of a Lobster has the same two-sided or bilateral 

 symmetry as that of a Vertebrate, and there is a clear distinction 

 between front (anterior) and back (posterior) ends, upper (dorsal) 

 and lower (ventral) surfaces, and right and left sides. The body, 

 too, is segmented, or divided into a number of similar parts from 

 before backwards, as in, say, a Lancelet. This is evident in the 

 Lobster's tail. It must not, however, be hastily assumed that 

 a segment of a Lobster is the exact equivalent either of a Verte- 

 brate segment or a segment in an Invertebrate from another 

 group. 



Now come a number of important differences. A large 

 number of jointed limbs are present, arranged in pairs, while a 

 Vertebrate has at most two pairs of limbs, though these may 

 differ in nature in different animals, and in the simplest case, 

 that of fishes, are unjointed fins. The limbs are modified for 



