varying between wide limits, approaching not only nearly allied 

 species, but even species which in their typical development are 

 quite different. This may, not seldom, depend partly on local 

 relations, partly and more frequently caused by attack of animals, 

 as saxicavous sponges, or worms and especially boring-muscles, 

 destroying particularly the lower or inner parts, or numerous other 

 animals fastened to or living on the plant. The consequence of 

 it is often an alteration in the development of the individuals in 

 their struggle for existence by a tendency to overlap the animals, 

 or other external objects, and thereby assuming much varying 

 forms. 1 ) Individuals attacked b} r boring-muscles may occasionally 

 be quite destroyed thereby, that their growth has not been able 

 to keep pace with the destructive work of the muscles. I have 

 seen numerous individuals, especially densely branched ones lying 

 loose on the bottom, in their inner parts so filled with boring- 

 muscles, that only a thin peripherical portion has been left, forming 

 what may be called a common testa around the colony of muscles. 

 Together with these muscles is also to be found numerous per- 

 forated and broken pieces or branches of the plant. By a feeble 

 pressure, a push, or by the continued labour of the muscles the 

 remaining peripherical portion gets broken, and the whole plant 

 collapses. In specimens more attacked towards the one than the 

 other side an opening often is produced, through which other not 

 perforating muscles and other animals push their way into the 

 plant. However, such specimens apparently seldom get quite 

 destroyed, as the other parts of the plant becomes more vigorously 

 developed, but, on the contrary, they sometimes assume peculiar 

 forms much differing from the typical ones. Most of the specimens 

 that I have examined have, moreover, been infested with numerous 

 perforating algas, which in general rather destroy chiefly the lower 

 layers of tissue. Growing in shallow water, from or a little below 

 extreme low-water mark to a depth of about 2 — 3 fathom, in 

 narrow sounds with somewhat strong currents, or else in places 



a ) In one locality especially I met with numerous killed or dying muscles 

 (Mytilus modiolus) caused thereby, that the Lithothamnia encompass the 

 shells. Cp. Contrib. II, p. 3, and Alg. og Musi. p. 17. 



