PHYSIOLOGY OF THE INVERTEBRATA. 245 



But this power of secreting exoskeletons is not confined to 

 the Molhtsca ; even in the Protozoa calcareous and siliceous * 

 -exoskeletons are secreted by the cells acting in the first 

 instance on the calcium carbonate and silica dissolved in the 

 water in which these animals live. " The hard protective 

 skeletons in all Invertebrate Metazoa, except the Porifera, 

 the Adinozoa, the Uchinodermaia, and the Tunicata, are 

 cuticular structures, which may be variously impregnated 

 with calcareous salts formed on the outer surface of the 

 epidermic cells. In the Porifera, the calcareous or siliceous 

 deposit takes place within the ectoderm itself, and probably 

 the same process occurs, to a greater or less extent, in the 

 Adinozoa. In those Tunicata which possess a test it 

 appears to be a structure sui generis, consisting of a gela- 

 tmous basis excreted by the ectoderm, in which cells 

 detached from the ectoderm divide, multiply, and give 

 rise to a deposit of cellulose." 



" In the Adinozoa and the Hchinodermata the hard skele- 

 ton is, in the main, though perhaps not wholly, the result of 

 calcification of the elements of the mesoderm. In some 

 Molluscs, portions of the mesoderm are converted into true 

 cartilage, while the enderon of the integument often becomes 

 the seat of calcareous deposit." 



Besides the various glands, cells, and devices secreting 

 and excreting calcareous, siliceous, and gelatinous materials, 

 integumentary glands and aggregations of glands may also 

 acquire a relation to the acquisition of food ; as, for in- 

 stance, in the spinning glands of spiders. Finally, mucous 

 glands are very widely present in the integument of animals 

 which live in damp situations (snails, &c.), and in water 

 (Annelids, Meduscc, &c.). 



At this point we intend to describe in detail the excretory 

 or renal organs, as well as to allude to certain important 



* See also Murray and Irvine's paper in Frop. Hoy. Soc. Edinh,, vol. 18, 

 p. 229 (1891). 



