ANNELIDA. 67 



many enemies. The anterior part of the body is specially sensi- 

 tive to light, which perhaps, as Darwin suggested, passes through 

 the skin and acts directly on the cerebral ganglia. It has, however, 

 been stated that all the pigmented regions of the body are affected 

 by blue, violet, and ultra-violet rays. The sense of hearing is 

 entirely absent. 



DEVELOPMENT (Fig. 20). 



The egg-capsules of the Earthworm are deposited during spring 

 and summer in damp earth, a few inches from the surface. They 

 are olive-green in colour, spindle-shaped, and about J of an inch 

 long. Each capsule contains a mass of slimy albumen and several 

 eggs, of which, however, only one usually comes to maturity. 



Fertilization takes place within the capsule, after which the 

 oosperm undergoes continued cell-division (cleavage, segmentation) 

 to form a hollow sphere (blastula, blastosphere) with a cellular 

 wall. The sphere is then converted into a double-walled bag or 

 gastrula by inpushing of its wall. The inner layer of the gastrula 

 is endoderm, the outer ectoderm. At the same time the third 

 germinal layer, mesoderm, begins to develop by division of two 

 large cells (mesoblasts) found near the posterior end of the body. 

 The embryo gradually becomes more and more elongated, the 

 various organs being at the same time differentiated from the 

 three germinal layers. The surrounding albumen is used as food, 

 and one embryo develops more rapidly than the others which 

 it utilizes in the same way. In the case of capsules which are 

 kept in the laboratory, hatching takes place in from two to three 

 weeks. 



1. Cleavage (Segmentation). This is complete (holoblastic) 

 and irregular, the entire oosperm dividing, but the resultant cells 

 being of unequal size. The divisions take place in a regular way 

 (B, C) until a 7- or 8 -celled embryo is produced, consisting of 

 2 large, 2 medium-sized, and 3 or 4 (probably ectodermic) small 

 cells. Beyond this there seems to be no definite order. The com- 

 pletely segmented oosperm is a blastula (blastosphere) containing 

 a large segmentation-cavity (s.c) (blastocoele), bounded on one side by 

 small and on the other by large cells, which respectively become 

 ectoderm and endoderm (D, E, F). Two large cells (mesoblasts) 

 are also distinguishable (E, M) from which the whole of the 



