INVERTEBRATA. 345 



are formed by cell-division : each of these is a cell, whose 

 nucleus soon undergoes repeated division, the nuclei arrang- 

 ing themselves at the surface. Corresponding to each 

 nucleus a wart-like swelling, the spermoblast, appeal's on 

 the surface, and soon the spermospore has the appearance 

 of a blackberry. Each little protuberance eventually 

 elongates and becomes the tail of a spermatozoon, with the 

 nucleus for its head. The central mass of protoplasm 

 (sperm-Uastophor) is apparently wasted. 



When the ova are ripe, the clitellum secretes a cylindrical 

 COCOOn, out of which the worm wriggles itself backwards. 

 As the fourteenth segment passes through the cocoon, the 

 ova are laid in it; and as the tenth passes through, the 

 " foreign " spermatozoa are shed into it. The cocoon is 

 elastic and closes when the worm has finally got out of it. 

 In this cocoon the ova develop into young worms. It is 

 unnecessary to detail the stages of development, for the 

 earthworm is a specialized member of its group, and without 

 a comparison with that of more typical marine worms its 

 development is as uninstructive as would be that of the 

 rabbit without a knowledge of fowl, frog, and Amphioxus. 

 It will be sufficient to say that a gastrula stage is passed 

 through; epiblast, mesoblast, and hypoblast are formed 

 much as in craniate Vertebrates, and give rise to the same 

 tissues respectively as in them ; that the coelom is formed 

 by splitting of the mesoblast ; the central nervous system 

 by a paired thickening of the epiblast, without neural folds 

 or groove ; and that the blastopore, instead of being postero- 

 dorsal, is ventral, and between the positions of the future 

 stomodseum and proctodseum, there being nothing re- 

 sembling a neurenteric canal. These points illustrate the 

 fact that only in the very broadest features of development 

 does Lumbricus agree with the Vertebrates : as soon as we 

 pass to the features which come second in importance, 

 striking differences at once show themselves. 



9. Systematic Position. Comparing Lumbricus with 

 Hydra, it is evident that we have a much higher organiza- 

 tion. The presence of a definite coelom (cf. figs. 175 and 

 3.95, A) of well-defined tissues other than epithelia, of elaborate 



