8 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



are ringed or segmented, from those which are unsegmented. 

 The former are often called Annelids, and include : — 



Chsetopoda, or Bristle-footed worms, e.g., earthworm 

 and lobworm ; and 



Hirudinea, or Leeches ; and some smaller classes. 



Unsegmented " Worms." 



These differ from the higher " worms " in the absence of 

 true segments and appendages, and resemble them in their 

 bilateral symmetry. The series includes Turbellarians or 

 Planarians ; the parasitic Trematodes or Flukes ; the para- 

 sitic Cestodes or Tapeworms ; the Nemerteans or Ribbon- 

 worms ; the frequently parasitic Nematodes or Thread- 

 worms ; and several smaller classes. 



As to certain other forms, such as the sea mats (Polyzoa 

 or Bryozoa), the lamp shells (Brachiopoda), and the worm- 

 like Sipunculids, it seems best, at this stage, to confess that 

 they are incertie sedis. 



But the general fact is not without interest that in the 

 midst of the well-defined classes of Invertebrates there lies, 

 as it were, a pool from which many streams of life flow, for 

 among the heterogeneous " worms " we detect affinities with 

 Arthropods, Molluscs, Echinoderms, and even A^ertebrates. 



At this stage we may notice that in all the above fomis the typical 

 symmetiy is bilateral {in Echinoderms, the radial synimetr)^ belongs only 

 to the adults) ; that in most types a body cavity or ccelome is developed ; 

 that the embryo consists of three germinal layers (external ectoderm or 

 epiblast, internal endoderm or hypoblast lining the gut, and a median 

 mesoderm or mesoblast lining the body cavity). In the next two classes 

 (Ccelentera and Sponges) the conditions are different, as may be expressed 

 in the following table, though it is open to question whether the contrast 

 is quite so great as it seems : — 



Sponges and Ccelentera. Higher Animals (Ccelomata). 



There is no body cavity. There is but ' There is a body cavity or cctlome between 

 one cavity, that of the food canal. ' the food canal and the walls of the 



body. But this is often incipient, or 

 1 degenerate. 



There is no definite middle layer of cells There is a distinct middle layer of cells 

 (mesoderm), but rather a middle jelly (mesoderm) between the e.\ternal 



(mesoglo:;a). ectoderm and the gut lining endo- 



derm. 

 The radial symmetry of the gaslrula em- The longitudinal a.\is of the adult does 

 bryois retained in the adult, and the not correspond to the long ax is of the 



longitudinal (oral-abor.al) axis of the gastrula embr^'o. 



adult corresponds to the long axis of 

 the gastrula. 



