122 THE INVERTEBRATA 



to the principal external organs of sense, and to the nervous system 

 (in nearly all cases: a part of the nervous system of the Echino- 

 dermata is remarkable in being developed from the peritoneum and 

 therefore mesodermal). 



The nervous system was no doubt primitively subepithelial, having 

 arisen by specialization of cells of the epithelium for the transmission 

 of impulses due to stimuli received upon the epithelial surface. In 

 a number of cases (Coelenterata, some Annelida, Echinodermata, 

 Enteropneusta, etc.) it remains in that position, but usually it is in 

 a deeper and more protected situation. The central nervous system 

 arose as a condensation of the primitive subepithelial nerve plexus 

 which took place in different positions in different animals. In those 

 which have a long axis it has the form of cords along that axis. The 

 cords may be paired or unpaired, lateral, ventral, or dorsal. Anteriorly 

 they pass into an enlargement which constitutes a " brain " or cerebral 

 ganglion. In the Chordata the central nervous system is hollow; its 

 removal from the surface of the body having taken place not, as usual, 

 by separation from the epithelium, but by the folding-in of the strip 

 of epithelium with which it is connected and which still remains to 

 line its cavity. A similar condition is seen in some echinoderms. 



Within the massive layer of mesoderm, channels are necessary for 

 the transport of food, excreta, hormones, germ cells and so forth, 

 and often there must be spaces to give play to movements of the 

 viscera. Such facilities are provided by the following two systems of 

 cavities, of which either or both may be present: 



(a) The primary body cavity, sometimes known as the haemocoeky 

 lies in the mesenchyme, and is to be regarded, morphologically, as 

 representing that part of the blastocoele which is not obliterated by 

 the mesenchyme cells or by a solid matrix or fibres secreted by them. 

 Its fluid contents, containing free mesenchyme cells ("corpuscles"), 

 are the blood and lymph, and it has usually the form of a branching 

 system of vessels ("vascular system") through which the fluid is 

 caused to circulate by the contraction of muscular fibres in the wall 

 of some portion of it which is known as a heart. In some cases, how- 

 ever, the haemocoele forms large "perivisceral" sinuses around the 

 internal organs. It never contains germ cells or communicates with 

 the exterior. 



(b) The secondary body cavity or coelom is from the first completely 

 surrounded and separated from the blastocoele by the mesothelium, 

 which is derived, as we have seen, from the endoderm. This cavity 

 has various forms, but is rarely tubular and never possesses a heart. 

 Usually it constitutes one or more large perivisceral spaces around 

 the heart, alimentary canal, and other organs. It will be noted that 

 the perivisceral cavity which surrounds the internal organs of most 



