228 The Study of Animal Life PART m 



As sponges showed tissues in the making, so among Stinging- 

 animals organs begin eyes and ears, nerve -rings, and special 

 reproductive structures. The zoologist has much to learn in regard 

 to the alternation of hydroid and medusoid in one life-cycle, the 

 division of labour in Hydractinia and other colonies, and the 

 meaning and making of a skeleton. Nor can we forget the long 

 past in which there were ancient coral reefs, and types of coral 

 hardly represented now, and strange Graptolites whose nature we 

 do not yet clearly understand. 



We begin the series of many-celled animals with Sponges and 

 Ccelenterates, partly because they are on the whole simplest, but 

 more precisely because their types of structure are least removed 

 from that two-layered sac-like embryo .or gastrula which recurs in 

 the life-history of most animals, and which we have much warrant 

 for regarding as a hint of what the first successful many-celled 

 animals were like. The Sponges and Ccelenterates differ from the 

 higher animals : (l) In retaining the symmetry of this gastrula, in 

 being like it radially symmetrical, and in so growing that the axis 

 extending from the mouth to the opposite pole corresponds to the 

 long axis of the embryo; (2) in being two -layered animals, for 

 between the outer skin and the lining of the internal food-cavity 

 there is only a more or less indefinite jelly instead of a definite 

 stratum of cells ; (3) in having only one internal cavity, instead of 

 having, like most other animals, a body-cavity within which a dis- 

 tinct food-cavity lies. 



3. "Worms." This title is one of convenience, without 

 strict justification. For there is no class of "worms," but an 

 assemblage of classes which have little in common. "Worm" 

 is little more than a name for a shape, most of the animals so 

 called differing from anemones and jellyfish in having head, tail, and 

 sides. The simplest worms were apparently the first many- 

 celled animals to move persistently head foremost, thus acquiring 

 distinct bilateral symmetry, and a definite nervous centre or brain 

 in that region which had most experience the head. In our 

 survey we are helped a little by the fact that many consist of a 

 series of rings or segments, while others are all one piece or unseg- 

 mented. It is generally true that the latter are in structure simpler 

 and more primitive than the former. 



ist Set of Worms. Plathelminthes or flat worms, ist 



Class. Turbellaria or Planarians. These are small worms, 

 living in the sea or in fresh water, or occasionally in damp earth, 

 covered externally with cilia, very simple in structure, usually 

 feeding on minute animals. The genus Planaria, common in fresh 

 water ; green species of Vortex and Convoluta, which are said by 

 some to owe their colour to minute partner algae ; Microstoma, which 



