THE STRUCTURE AND LIFE-HISTORY OF SPONGES. 231 



slides, I can now sum np our knowledge on this subject as follows: — 

 In the circumference of the inhalent pores, or scattered irregularly 

 over the outer surface, or in the membranes which traverse the 

 lacunar cavities in the interior of the sponge, or also round the 

 pores in the cribriform plates which cover the pseudoscula of 

 certain tubular species, nerve-cells are found. Thej' are rarely 

 single, much more frequently in groups of from three to six. 

 In Grantia high and slender conical processes are found on the 

 outer surface. In the widened basal portion of these, oval cells 

 are situated from which irregular processes radiate. One of these 

 processes is much larger than all the others, and extends up- 

 wards in the form of a fine thread to the distal end of the organ. 

 The other processes of the cell extend downwards. In each organ 

 several such cells are found. These organs are the Palpocils of 

 Stewart, who, not noticing the fact that they are polycellular, gave 

 them that name. Possessing this extraordinary and most im- 

 portant peculiarity, I have termed them Synocils. It is remark- 

 able that these structures have escaped observation hitherto, but 

 it appears highly probable that the sponge has the power of 

 retracting them, so that they are visible only under exceptionally 

 favourable circumstances. It is very likely that the groups of 

 cells previously observed by me, and described as sensitive, are 

 nothing else than retracted Synocils. 



The nervous system appears to be most highly developed 

 in Eiispongia canaliculata, where continuous zones of nervous 

 tissue — sensitive cells above and ganglion cells below — are 

 observed surrounding certain lacunose areas below the surface. 



The collections made during the voyages of the 'Alert' and 

 ' Challenger,' as well as my own labours in the Australian Seas, 

 have extended our knowledge of the geographical distribution of 

 sponges so that we are now able to form a general idea of it. 

 Sponges occur in all seas. Those living at great depths are 

 mostly cosmopolitan, while those which occur in shallow water are 

 very different in various localities. The Tropical and Polar zones 

 possess not nearly so rich a sponge fauna as the Temperate zone. 



Besides thirteen families of Hexactinellida and Lithistida, 

 which, being deep-sea sponges, are mostly cosmopolitan, there are 

 thirty-three families of marine sponges which live mostly in 

 shallow water, the distribution of which I will discuss. Of these, 

 twenty-five are cosmopolitan, two occur in the North Atlantic, and 



