ANNELIDA 81 



of invaginated ectoderm that is to say, the mouth, buccal 

 cavity, and the pharynx this is the stomodeum. There is 

 also a small proctodeum posteriorly. All the rest of the canal 

 is formed of endoderm and constitutes the enteron. 



Body Cavity. The secondary body cavity, or coelom, is 

 highly developed in the earthworm. It has been exposed 

 in making out the parts of the alimentary canal. The cavity 

 is continuous, but it is interrupted by septa which divide the 

 cavity into nearly separate compartments corresponding with 

 the external rings. The first septum is the posterior one of 

 the fourth segment, and the others occur without interruption 

 to the last segment. The septa are perforated ventrally 

 where the nerve cord passes through them. The body cavity 

 is lined by a thin epithelium or peritoneum. The peri- 

 toneum is disposed in a parietal layer internal to the longi- 

 tudinal muscles of the skin, and each septum is made by the 

 layer folding transversely inwards as a double layer. These 

 diverge from one another to envelop the alimentary canal, 

 and this tubular part is called the visceral layer. The two 

 layers of each septum fuse to form the intercommunicating 

 foramen round the nerve cord. The septum is therefore a 

 transverse mesentery. It has to be observed that none of 

 the organs occurs in the cavity except certain of the generative 

 organs, as will be stated later. The coelom is distended by a 

 fluid, the coelomic fluid, which has amoeboid corpuscles or 

 leucocytes floating in it. The visceral layer of the peritoneum 

 throughout the length of the intestine is tinged yellow, and is 

 thickened from the presence of yellow cells. It was long 

 supposed that these cells took a share in absorbing the food 

 from the enteron, but their function is evidently to abstract 

 waste material from the endoderm and the blood and to cast 

 this into the coelomic fluid for excretion. In the Oligochaeta 

 they are associated with the dorsal vessel and its factors, and 

 they are found also in the Polychaets. 



The primary body cavity occupies the little amount of 

 space between the coelom and the alimentary canal on the 

 one hand and the ectoderm on the other. It expands in 

 the anterior end of the earthworm. Elsewhere it forms a 

 connective-tissue system penetrating between the peritoneal 



