230 MANUAL OF ELEMENTARY ZOOLOGY 



In comparing the body of an earthworm with those of 

 the other examples of the Metazoa that we have 

 Mesoderm, studied, it will be seen that in one respect of 

 Hsmooosie. importance it resembles the frog rather than 

 Hydra. The body of Hydra consists of two 

 epithelia — the ectoderm and endoderm — with only a 

 structureless lamella between them. In a frog or an earth- 

 worm these epithelia reappear as the epidermis and the lining 

 epithelium of the gut, but between them is a great mass of 

 tissue which comprises the skeletal tissues, muscles, excre- 

 tory and generative organs, and so forth. These tissues are 

 together known as the mesoderm, and animals which possess 

 this third layer are known as Triploblastica, while those, 

 like Hydra, which possess only two are Diploblastica. 

 Both in the earthworm and in the frog the mesoderm 

 contains cavities of two kinds, the primary body cavity or 

 hozmocale or blood vessels, and the " true " or secondary body 

 cavity or ccelom. The functions of the ccelom are threefold, 

 (i) It forms a perivisceral cavity, which surrounds the prin- 

 cipal viscera and gives room for their movements. (2) From 

 its walls are derived the generative cells. This is clearly 

 seen in the case of the ova, which are shed into the peri- 

 visceral cavity, but it is less clear in the case of the sper- 

 matozoa, because these are passed into special vessels. 

 (3) It is concerned in excretion. In an earthworm, where 

 the yellow cells of its walls form excreta which are removed 

 by the nephridia, this is more obvious than in the frog, but 

 the latter possesses in the tadpole stage (and, according to 

 some authorities, also in the adult) funnels which, like those 

 of the nephridia of the earthworm, pass ccelomic fluid into 

 the excretory tubules, where it serves to wash out the waste 

 products. The hsemoccele is a system of spaces of more 

 complex form than the ccelom, and rarely (p. 249) peri- 

 visceral. Its function is to contain the blood and lymph. 

 A blood-vascular system is made necessary in most triplo- 

 blastic animals by the presence of the great mass of internal 

 tissues which constitute the mesoderm. 



Another feature of the morphology of an earthworm to 



which attention must be called here is its 



segmentation. We have seen that merism or 



the repetition of parts is universal among animals. In an 



