66 LOWER INVERTEBRATES. 



pink. The fibres of the skeleton are light brown, and friable when dry. The fleshy 

 parts disintegrate so readily after death that these sponges cannot be kept for any 



length of time even in the strongest alcohol. Tuba is 

 another genus of this group. No fossils are known. 



StJB-OeDER II. — ECHDSrONEMATA. 



The spicules of the Echinonemata are of two or 

 more kinds. The simple smooth or double-pointed 

 ones usually lie in the fibre, the rough single-pointed 

 ones, with a more or less expanded base, stand out 

 from the keratode, leaving the point bare. Defen- 

 sive surface spicules are often present. 



This sub-order is represented on our coasts by 

 Microciona prolifera, which grows abundantly in the 

 pools and tideways south of Cape Cod. When in still 

 water and on a smooth surface it forms a thin, smooth 

 sheet, but under other conditions it tends to grow up- 

 right, and form branching masses a few inches in 

 height. The color is a bright orange red, producing 

 rich effects in pools where much of it is present. 

 No fossil Echinonemata are known. 



Sub-Order III. — Monactinellinje. 



In this group the fibres are composed of straight 

 silicious spicules, while the amount of keratode is very 



ma.it.-Tubamynnthiformu. flight. The most common form is the Crumb-of- 



bread Sponge, Halichondria panicea, which has a 

 world-wide distribution, and occurs plentifully in a dried state upon our beaches. It is 

 almost as light as dried bread, and when well bleached is very white. 



Another form, Suherites compacta, occurs on the coast south of Cape Cod, and is 

 the only form in that region which is able to live upon the shifting sands. The pores 

 are so small, and the structure so dense, that the sand cannot obtain an entrance, while 

 its lightness keeps it from being buried. Specimens securely anchored have been 

 found, and evidently the usual free condition is an acquired adaptation to a habitat on 

 a sandy bottom. They are washed ashore in considerable numbers, and so fine and 

 homogeneous are their spicules that the skeletons are said to have been formerly used 

 for polishing silver. It grows in flattened masses of a yellow color, but the skeleton 

 when bleached is white. Another species of this genus also frequents the sands north 

 of Cape Cod, but finds more congenial accommodation on the shell of a species of 

 gasteropod, nearly all of which, in certain localities, bear a sponge. 



Some of this group have accustomed themselves to lead a life of borers, and though 

 not successful with hard rocks they are very destructive to the shells of various 

 molluscs, and even to limestone and marble. Cliona sulphurea, a very common form, 

 is the most remarkable of these borers. It penetrates and excavates chambers in the 

 shell of a mussel for example, and then, after causing the death of the animal, it will 

 entirely enclose and resorb what is left of the shell. Not content with this conquest it 



