Metazoa Lumbricus. 249 



stage of its development, and is comparable to the planula 

 formed by Hydra and Obelia. The small cavity in the in- 

 terior of the gastrula is the primitive alimentary canal, or 

 archenteron ; its slit-like mouth forms the blastopore, after- 

 wards to become the true mouth. 1 



By this time the mesoblast has made its appearance by 

 the multiplication of two large cells, covered over by epiblast 

 and lying close to the blind end of the gastrula. It forms a 

 plate of cells on either side of the body from end to end of 

 the embryo. These plates become segmented into blocks, 

 known as mesoblastic somite?, each of which becomes 

 hollowed, forming a layer of mesoblast surrounding the 

 enteron and a layer applied to the epiblast. This is known 

 as metameric segmentation to distinguish it from the seg- 

 mentation of the egg. The two layers of mesoblast are 

 of great importance in embryology, and are named the 

 splanchnopleure, or visceral, and somatopleure, or body 

 layers respectively. Between the two layers there lies a 

 space, the ccelom, or body- cavity, subdivided into as many 

 small chambers as there are mesoblastic somites, and 

 bounded in front and behind by partitions formed by the 

 fusing of the anterior wall of each mesoblastic somite with 

 the posterior wall of that immediately in front. The anus 

 is formed late in development by a tucking in of the pos- 

 terior region of the body. The enteron is thus placed in 

 communication with the exterior at both ends. 



The several organs found in the ccelom are formed from 

 the rapidly increasing mesoblast, save the nervous system, 

 which is formed out of epiblast and sinks into the mesoblast 

 in the course of subsequent development and growth of the 

 embryo. 



In concluding our survey of Lumbricus we may note for 

 future reference the importance of the segmented condition, 



1 It will be afterwards seen that the mouth is usually formed in an 

 entirely different manner (p. 262). 



