412 ELEMENTS OF BIOLOGY 



of the shape o£ colonies of certain Protozoa, for example, volvox 



(p. 89). 



(2) The gastrula of Metazoa consists of a two-layered sac, or 

 some modification thereof, the inner layer destined to become the 

 digestive system and its attendant structures, the outer to become 

 parts of the integument and the entire nervous system. The adult 

 ccelenterate, hydra, is constructed on a similar plan, the inner 

 layer being essentially digestive in function, and the outer layer 

 containing cells specialized to receive and transmit stimuli. 



(3) In all Metazoa the third germ layer, the mesoderm, gives 

 rise to essentially the same types of adult structures, namely, cir- 

 culatory, supporting, muscular, excretory (usually), and reproduc- 

 tive systems. 



(4) The developmental stages of some phyla are very similar.. 

 For example, Mollusca, Annelida, some Molluscoidea, and Geph- 

 yrean worms pass through a stage in which the larvae are free- 

 swimming forms that closely resemble each other, known as 

 trochophore larvae (Fig. 76). 



(5) The embryos of higher animals pass through a condition in 

 which the character of segmentation is very distinct, although as 

 development proceeds it becomes obscured to a considerable extent. 



(6) Within a phylum, the basic organization of the various organ 

 systems develops in much the same way, regardless of wide or 

 narrow divergencies between adult types within the group. For 

 example, the steps in the formation of the digestive system are 

 very similar throughout the sub-phylum Vertebrata. 



Specific examples of evolutionary records in the embryo are 

 literally innumerable; in the embryo of cattle the formation of 

 teeth which disappear before birth; the pre-natal hair coat in the 

 human embryo; the neuropore that forms as the neural tube closes 

 in the vertebrate embryo, which is identical in position with the 

 neuropore of adult Amphioxus (lower chordate, p. 115) that per- 

 manently connects the neural canal of the adult with the surround- 



