8 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



DIAGRAM I. 



Chief Types of Structure. 



C. Lowest is a single cell, an Anlmba, with irregular outflowings of 

 its living matter, with a nucleus (n) or cell-kernel, with two con- 

 tractile vacuoles, and with some large granules in its cell-substance. 

 Bl. Next is a ball of cells, such as is formed when an ovum or egg- 

 cell divides ; or it may represent more diagramraatically one of 

 the exceptional multicellular Protozoa. 

 G. Third is a gastrula, in section, showing two layers of cells, the 

 outer ectoderm or epiblast, the inner endoderm or hypoblast, and 

 the central cavity. 

 Next lies an unsegmented worm, with a simple blind gut, with an 

 anterior patch of nerve-cells from which lateral nerve-cords run fore 

 and aft. 



Above this there is a diagram of a segmented worm, showing a food- 

 canal (with a darkened mid-gut), a ventral nerve-cord with ner\-e-centres 

 or ganglia in each segment, and a dorsal brain. There are also simple 

 feet. 



To the right side is a diagram of a starfish, in which the radiating 

 nervous system is emphasised. 



To the left side is an Arthropod, e.g. , a Myriopod, with jointed legs, 

 a dorsal heart, ventral nerve-cord, dark mid-gut, etc. 



To the right again is a rough copy of Ray Lankester's ideal Mollusc, 

 showing the dorsal heart, the ventral "foot," the coiling food-canal, 

 and other structures which can be readily identified from the chapter 

 on Molluscs. 



Highest is a very di^rammatic ideal vertebrate, showing the dorsal 

 nervous system, the subjacent notochord, the gut with gill-slits, the 

 ventral heart, and the segments. 



