52 LOWER INVERTEBRATES. 



is permeated by numberless tubes, but these can be reduced into two systems, one lead- 

 ing from the interior outward, and the other leading from the external surface toward 

 the interior. The first or internal system is comjjosed of several large trunk tubes, 

 largest interiorly, but branching and becoming smaller as we approach the exterior. 

 The outer surface of the sponge is ornamented with projecting bunches or ridges of 

 threads. Between these projections there are numerous depressions, the bottoms of 

 which are perforated by openings of medium size, which we can follow as tubes lead- 

 ing into the interior by examination of the cut surface of the section. These are the 

 tubes of the external system. They often terminate abruptly, but here and there are 

 divided into branches, and we can see that they really diminish in size towards the in- 

 terior. Not infrequently these tubes may be traced directly into the trunks of the 

 internal system, but in this case, their walls are thickly set with the openings of small 

 tubules which lead into systems of tubes diminishing in size internallj-, and therefore 

 belonging to the external system. The dried skeleton looks as if there was no room 

 for fleshy material between the meshes, but the increase in size upon wetting a sponge 

 shows that when in the natural element and fully expanded there is plenty of room 

 between the threads for all the organs wc have to describe. 



The surface of the living commercial sponge is of a dark color, and some sjjecies, 

 were they smoothei', would remind one of a piece of beef liver. On the ujiper surface 

 we can see large crater-like openings as in the skeleton, but the surface is otherwise 

 quite different. The tufts of fibres and the depressions between them, which are so 

 marked in the skeleton, are more or less covered with a skin which conceals all the 

 cavities and channels. The tufts, however, do show themselves as slight prominences, 

 while the skin over the intervening dej)ressions is smooth and perforated by groups of 

 holes. These small holes may be opened or closed at the will of the animal, and when 

 open they serve to admit water freely to the external or supply sjstem of tubes. 

 These openings may in many sponges entirely disapijear, and new apertures be formed 

 when needed. This faculty has, however, been greatly exaggerated. 



The supierficial cavities are lined Avith a smooth skin, lighter in color than that of 

 the exterior, while the sides and bottom are perforated by small holes, the openings of 

 the tubules which line the skeletal tubes of the external system and form the fleshy 

 canals of the supplj' system. These tubes are lined witli a light colored skin and 

 braucli as they descend into the interior. The tijjs of the minute branches expand into 

 globular sacs. These little enlargements, the ampullre, open in turn, into small fleshy 

 tubules which line the internal system of tubes of the skeleton. They constitute what 

 may be called a drainage system, and instead of growing less, they inci-ease in size as 

 they go inward, and by uniting with other similar tubes, they form lai-ger and larger 

 branches until they finally open into one of the central trunks. 



These sieve-like openings, the superficial hollows, and the supply system act as 

 feeders, bringing water loaded with nutriment to the ampullae or digestive sacs. After 

 digestion the refuse is jiassed out of the ampullae into the internal system and thence 

 into the large central trunks which finally open on the outside of the sponge in large 

 crater-like orifices. In some sponges these two systems of canals are not distinguish- 

 able and there is but one outlet to the ampullae. 



The outermost covering of the body is an extremely delicate membrane composed 

 of a single layer of flat cells, giving a peculiar shade of purple bloom to tlie living 

 sponge, but being easily abraded by rough handling. This layer is the ectoderm, and 

 is continuous at the edges of the craters with a somewhat similar layer, lining all of 



