248 THE PHILOSOPHY OF BIOLOGY 



independent existence. The Porifera, and all the other 

 phyla, include organisms the bodies of which are made 

 up of aggregates of cells. In the Porifera the cells, 

 which are specially modified in structure, are arranged 

 to form the internal walls of a " sponge-work " the 

 cavities of which open to the outside by series of pores 

 through which water is circulated. The bodies of the 

 Ccelenterates are typically sacs formed by a double 

 wall of cells — endoderm and ectoderm. This sac opens 

 to the exterior by a single opening, or mouth, sur- 

 rounded by a circlet of tentacles, and its cavity is 

 the only one contained in the body of the animal. The 

 Platyhelminth worms are animals the bodies of which 

 are also composed of ectodermal and endodermal tissues, 

 between which is intercalated another mesodermal 

 tissue. They have a single digestive sac or alimentary 

 canal opening to the exterior by means of a mouth 

 only ; and they all possess a complex, hermaphrodite, 

 reproductive apparatus. In all the other phyla there 

 are also three principal layers or kinds of tissue, but in 

 addition to the cavity of the alimentary canal there is 

 also a body cavity, or ccelom, which is contained in the 

 mesodermal tissues. The Echinoderms are such coel orn- 

 ate animals, but the alimentary canal now opens to 

 the exterior by means of both mouth and anus ; there 

 are separate systems of vessels through which water 

 and blood circulate ; the blood-vascular system of 

 vessels is closed to the exterior, the water-vascular 

 system being open ; and the integument is armed by 

 means of calcareous spines or plates. The Annelids 

 are animals with cylindrically shaped bodies, segmented 

 so as to form numerous joints. Each segment bears 

 spines or hairs or appendages of some sort, and also 

 contains a separate nerve-centre. The alimentary 

 canal opens externally by a mouth and anus, and there is 



