8 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



are ringed or segmented, from those which are unsegmented. 

 The former are often called Annelids, and include : 



Chaetopoda, 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 incertce 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 Vertebrates. 



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

 symmetry is bilateral (in Echinoderms, the radial symmetry belongs only 

 to the adults) ; that in most types a body cavity or coelome 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 

 (Coelentera 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 COELENTERA. 



HIGHER ANIMALS (CCELOMATA). 



There is no body cavity. There is but 

 one cavity, that of the food canal. 



There is no definite middle layer of cells 

 (mesoderm), but rather a middle jelly 

 (mesogloea). 



The radial symmetry of the gastrula em- 

 bryo is retained in the adult, and the 

 longitudinal (oral-aboral) axis of the 

 adult corresponds to the long axis of 

 the gastrula. 



There is a body cavity or coelome between 

 the food canal and the walls of the 

 body. But this is often incipient, or 

 degenerate. 



There is a distinct middle layer of cells 

 (mesoderm) between the external 

 ectoderm and the gut lining endo- 

 derm. 



The longitudinal axis of the adult does 

 not correspond to the long axis of the 

 gastrula embryo. 



