ANNULOSE I)IKFi:i;r.\ ; ;,| 



omy and development compel the nmrpli 

 t he whole of the Metazoa as modifications of one actual or 

 ideal primitive type, which is a sac with a double -llular 

 wall, in -losing a central cavity and open at one end. This is 

 what ilaeckel terms a Gastrwa. The inner wall of the MO is 

 the hypoblast (endodertn of the adult), the outer tin- rpiblast 

 (ectoderm). Between the two, in all but the very lowest 

 M lazoa, a third layer, the mesoblast (meaoderm of the adult i, 

 makes its appearance. 



In the Porifera, the terminal aperture of the gaktraea 

 becomes the egestive opening of the adult animal, and the 

 -itive apertures are numerous secondary pore-like aper- 

 tures formed by the separation of adjacent cells of th* 

 toderm and endoderm. The bo Jy may become variously 

 branched, a fibrous or spicular endoskeleton i> usually de- 

 veloped in the ectoderm, and no perivisceral cavity is de- 

 veloped. There are no appendages for locomotion <T jue- 

 hension ; no nervous system nor sensory organs are kn >wn to 

 exist; nor are there any circulatory, respiratory, renal, or 

 generative organs. 



In the Coelenterata, the terminal aperture of the gastnea 

 becomes the mouth, and, if pores perforate the body- walls, 

 they do not subserve the ingestion of food. There is no sep- 

 arate perivisceral cavity, bur, in many, an enteroccele or sys- 

 tem of cavities, continuous with, but more or less separate 

 from, the digestive cavity, extends through the body. Pre- 

 hensile appendages, tentacul ', are developed in L 

 A chitinous exoskeleton appears in some, a calcar. <!, 

 inous endoskeleton in others. There are no circulatory, re- 

 spiratory, or renal organs (though it is possible that certain 

 cells in the PbrpitOS, e. g., may have a uropoietic funeti 

 but special genital organs make their appear.ui> ,-, as do a 

 definitely-arranged nervous system and organs of sense. 



The lowest Tarbellaria are on nearly tin- same grade of 

 organization as the lower Ccelenterata, but the thick meso- 

 derm is traversed by canals which constitute a water-vase > 

 system. In the adult state these canals open, on the one * 

 into the interstices of the mesodermnl . and, on the 



other, communicate with the exterior. Th.-ir anal<>-\ to the 

 contractile vacuoles of the Infusoria on the one hand, and to 

 the segment al organs of the Annelids on the other, lead 

 to think that they are formed by a splitting <>f the mesoblast, 

 and that they thus represent that form of peri\ isc. ral c 

 which I have termed a schizoccete. A nervous system, con- 



