20 GENERAL CONCEPTIONS. 



we ascend the series, for we find that in the lowest fishes it persists well developed 

 throughout life ; in other fishes it disappears in part, in amphibia it disappears 

 almost completely, and in mammals it aborts entirely, and, so far as known, no 

 remnant of it normally persists in the adult. 



3. The hollow nervous system. This is found in vertebrates only, or in 

 animals which are closely related to vertebrates, so closely that to many natural- 

 ists they are included in the same sub-kingdom. The hollow nervous system is 

 enlarged in the region of the head, the enlargement constituting the brain. The 

 rest of it is of smaller size and constitutes the spinal cord. 



4. The limbs. There are two pairs, which are lateral extensions of the sur- 

 face of the body and acquire in their interior a skeleton by which they are sup- 

 ported and muscles by which they are moved. No homologous structures are 

 known in any invertebrate animal. 



5. The position of the mouth. The typical invertebrate mouth is sur- 

 rounded by the nervous system. For instance, in insects or in the jointed worms 

 (annelids) there is a brain, so called, above the mouth, and a strand of nervous 

 tissue running down on either side of the body past the mouth to join the gan- 

 glion on the lower side, thus completing a ring of nervous material through which 

 the oesophagus passes. In vertebrates, on the other hand, the mouth is not 

 enclosed by any oesophageal ring, and the entire nervous system is on one side of 

 the body and dorsal to the mouth. 



6. The division of the primitive body-cavity. The body-cavity in the em- 

 bryo is known by the comprehensive name of the coelom. It will not be possible 

 to acquire a clear idea of its division until the embryos are actually studied. It 

 forms many parts. Of these, there are two series, one on each side of the central 

 nervous system, which form cavities of what we designate as the primitive seg- 

 ments of the body. There are also two large divisions which extend from the 

 region of the head to that of the future pelvis, one division for each side of the 

 body. These two large parts are not divided into segments at all, though the 

 cavities of all of the segments are primitively connected with these two main 

 divisions. Comparatively early in the development the two main cavities be- 

 come connected with one another so as to constitute a single cavity to which we 

 apply the name of splanchnocele. The splanchnocele surrounds the heart of 

 the embryo, where we recognize it as the pericardial cavity, and it extends 

 through the future abdominal region, where we recognize it as the abdominal 

 cavity. The pericardial and abdominal regions of the cavity are separated from 

 one another in the embryo by a broad transverse partition which bears the name 

 of septum transversum. This septum in mammals becomes in the adult the 

 diaphragm. It is one of the most striking of all the morphological peculiarities 

 by which vertebrates are distinguished from invertebrates. 



