THE APODOUS HOLOTHURIANS 67 



of genera occur in the Arctic and Subarctic region, but the number of species 

 there is not large. Several of the tropical species occur in mangrove swamps, 

 where the sea-water is not as saline as on the reefs, and at least one of the 

 Philippine species is known to live ordinarily in distinctly brackish water. As 

 a rule, however, the Synaptids are quite susceptible to impurities in the water 

 and are to be found only where it is clean and well aerated. Three quite 

 distinct groups, in the matter of habitat, can be recognized, though they are 

 not sharply distinguished from each other. First, there are those species 

 which are really subterranean, living buried in the sand or mud and rarely 

 coming out from their burrows. A second group is made up of those species 

 which live under stones and in similar hiding places ; and third are those 

 species which live among corals or seaweeds, not concealed, except as their 

 color and form harmonize with their surroundings. The habitat of any given 

 species varies in different regions, so that subterranean species are often 

 found under rocks, and similar situations are sometimes frequented by species 

 which are ordinarily unconcealed. A certain amount of correlation between 

 color and habitat is seen throughout the family, for the brightly colored species 

 are those which live among corals, while the white, colorless, or inconspicuous 

 species are usually subterranean or at least lie concealed under stones. In 

 some cases this correlation is very striking; for example, Synaptula hydriformis 

 in Jamaican waters is reddish brown in color, in perfect harmony with the 

 seaweed it inhabits, while at the Bermudas it is more or less bright green, in 

 harmony with the Ulvaceae among which it lives. 



Very little is really known as to the food of the Synaptidas, but the alimen- 

 tary canal is usually found well filled with particles of sand and other inorganic 

 substances, which are taken in with the food. The tentacles of a living Synap- 

 tid are in almost constant motion, continually bending in toward the mouth, 

 and then curving outward again, and it is often possible to see particles of 

 solid matter being pushed into the mouth. In Synaptula hydriformis the food 

 is largely, if not wholly, vegetable, diatoms forming a conspicuous part of it, 

 and it is probable that the same is true of all those species which live in sea- 

 weeds. The food of Leptosynapta inhcerens is organic matter, taken in with 

 the sand through which it burrows, and apparently consists of both animal 

 and vegetable remains; probably all of the subterranean species and those 

 which live under stones 'feed on similar material. Whether the species which 

 live among corals are carnivorous has yet to be shown ; on a priori grounds, it 

 is highly improbable. 



Like all holothurians, Synaptids are very inactive animals. Unless irri- 

 tated by some exceptional external stimulus, their movements are slow and 

 uniform. There is no good ground for believing that they are more active 

 during the night than in daylight. Those forms which live under stones ap- 



