THE CCELOMATA INVERTEBRATA 191 



canal is far more highly developed, and possesses two external 

 openings, a mouth and an anus, and in addition sac-like glandular 

 outgrowths. It can be divided into an anterior part lined with 

 ectoderm, the stomodceum, a mid portion lined with entoderm, the 

 midgut or mesenteron, and a very small posterior portion also 

 lined with ectoderm, the proctodoeum. Owing to the presence of 

 the ccelom many parts of the body are separated from the alimentary 

 canal, and this is itself removed from the oxygen of the air, and so 

 we find a well- developed circulatory system for the distribution 

 of nutriment and for respiration. The gonads are inside the ccelom 

 of the worm, and consequently require genital ducts to convey their 

 products to the exterior. In Hydra both layers form a free surface 

 over which excretion can take place, but many layers in Lumbricus 

 are not free, so that it possesses a series of complex excretory organs 

 for its nitrogenous waste in the form of segmentally arranged 

 nephridia. Lastly, the diffuse network of nerve cells in Hydra 

 is replaced by a compact ventrally situated central nervous system, 

 connected on the one hand with the muscles, and on the other with 

 a number of sensory cells in the ectoderm. All of these points are 

 also characteristic of the higher ccelomates, save that in the Verte- 

 brata the concentrated nervous system is on the dorsal side of the 

 animal ; and although in the early development the excretory tubules 

 are segmentally placed, they lose this primitive arrangement in the 

 adult. Thus, just as Hydra represented a considerable advance on 

 the Protozoa, so Lumbricus in its turn is on a much higher plane than 

 the Ccelenterata, and, indeed, its metamerism and general plan, 

 although sometimes much modified, ca^nd) to be traced throughout 

 all the higher animals. 



Turning now to consider the development of Lumbricus, it is, 

 as might be expected, much more complicated than that of Hydra, 

 and we need only take notice of its main features. The egg is laid, 

 as we have seen, with others in a cocoon provided with a store of 

 nutrient fluid, and is spherical and fairly transparent, with a relatively 

 small amount of yolk. The segmentation or cleavage of the egg is 

 complete or holoblastic, that is to say, the lines of division cut the 

 original ovum up completely into separate cells, and a hollow sphere 

 of cells or blastula is formed. The cells of the blastula are of unequal 

 size, being divided into an upper set of small ones, the micromeres, 

 and a lower set much larger, the macromeres. All the cells continue 

 to divide, getting smaller in the process, save two neighbouring 

 macromeres near the equatorial line. These remain of large size, 

 and as they are destined to give origin to the mesoderm are variously 



