136 THE INVERTEBRATA 



union in various ways of nephridia with coelomoducts or other meso- 

 dermal elements (see p. 276). In such cases the nephridia acquire a 

 communication with the coelom, and excreta or germ cells may pass 

 from it through them. In other groups, as in some Crustacea, a 

 coelomoduct is supplemented or in great part replaced by an ecto- 

 dermal component, but there is no evidence that this component 

 represents a nephridium. 



iii. Ectodermal organs. The ectoderm gives rise to the epidermis 

 (epithelium which covers the body), to certain glands, to the ne- 

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

 system (in nearly all cases ; there are nerve cells under the endoderm 

 of certain coelente rates, and a part of the nervous system of the 

 Echinodermata is remarkable in being formed from the peritoneum 

 and therefore mesodermal). 



The epidermis with some underlying mesodermal connective tissue 

 known as the dermis constitutes the skin. In invertebrates it is 

 columnar or syncytial, in vertebrates it is stratified. In the lower 

 invertebrates its cells are usually ciliated, which was probably the 

 original condition. The cilia subserve locomotion, the taking of food, 

 or respiration (p. 139). When unciliated its protective function is 

 often increased by a cuticle. To it belong various glands, especially, 

 in naked epithelia, mucous glands whose secretion is protective, in 

 aquatic animals against parasites, in terrestrial against desiccation. 

 Others form cuticular structures, cement, poisons, etc. 



The nervous system was no doubt primitively situated immediately 

 below epithelia, having arisen by specialization of epithelial cells 

 for the transmission of impulses due to stimuli received upon the 

 surface — for the most part, presumably, upon the ectoderm. In 

 many cases (the Coelenterata, Echinodermata, Hemichordata, some 

 annelids, etc.) it remains there, but usually it is in a deeper, more 

 protected situation. All triploblastica possess a central nervous system. 

 This arose as a condensation of the primitive nerve-net of branched 

 cells, portions of which may remain unchanged. The central nervous 

 system was formed 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, the "brain" or cerebral 

 ganglion, connected with the principal organs of distant sense. In 

 Chordata the central nervous system is hollow, its removal from the 

 surface being not, as usual, by separation from the epithelium, but 

 by the folding-in of the strip of epithelium which it adjoins and 

 which remains to line its cavity. A similar condition is seen in some 

 echinoderms. From the central nervous system nerves proceed to 

 various parts of the body. 



