130 ESSENTIALS OF ZOOLOGY 



opening is formed. In the succeeding phyla the blastopore 

 is rotated forwards, but it still forms the mouth. It is only 

 in some of the Arthropods that the blastopore elongates so 

 as to give rise to the anus as well, but even so the mouth is 

 still formed from the blastopore. The mesoderm is at first 

 derived from both ectoderm and endoderm, but in the 

 Stomoporida the mesoderm becomes more strictly related to 

 the endoderm ; and in some of the Platyhelmia, the Eotifera, 

 the Annelids and Mollusca, investigations have been able to 

 show that certain of the cells produced during segmentation 

 form the ectoderm and its products, others the endoderm, and 

 a special cell the mesoderm. The latter is invaginated with 

 the endoderm. This relationship of the mesoderm to the lip 

 of the blastopore or to the invaginated endoderm is maintained 

 in the remaining groups. 



The blastopore of the remaining phyla is directly related 

 to the formation of the anus, and the mouth is formed inde- 

 pendently of it. These phyla may therefore be brought 

 together as Proctoporida. The Invertebrate representatives 

 may be grouped under the name Hydrocoela, and include the 

 Chaetognatha or arrow worms, Echinoderma or starfishes, 

 Pterobranchia and Enteropneusta. 



Endocrinology. Morphology is a fascinating study, but we 

 must not forget that the animal is still more interesting alive, 

 that the structures we expose by dissection and examine in 

 detail with the aid of the microscope are the structures which 

 make a living creature and by which it is kept alive. We 

 have had time to think of this to some extent, but the 

 exigencies of time and opportunity necessarily make our 

 laboratory work an enquiry mainly into structure. 



Every cell, whether it be free or a component of a cell 

 complex, is related to its environment, and the reactions of the 

 ectoplasm to the environment are conveyed by chemical and 

 mechanical changes to the endoplasm by the cell sap. The inter- 

 cellular sap, as we may term the fluid which circulates among 

 the mesenchyme cells of the primitive metazoon, similarly 

 communicates the effects of the environment to the whole 

 body, and mechanically as well as chemically^ for the pressure 

 it exerts by virtue of its condition with respect to food will 



