250 



CUTTLE-FISH, MOLLUSCA, ETC. 



which may be found wherever sea-weeds are to be 

 gathered, clustering their stems or spread out over 

 the fronds. It is held by naturalists that the ap- 

 pearance of sea-mats, sea-squirts, &c., in aquaria, 



Fig. 183. 



The Sea-fir (Sertularia abietind). 



is a proof that the water is in a healthy condition. 

 Another allied and abundant colony of marine ob- 

 jects all of them having affinities with the mollusca 

 in spite of their apparent differences may be seen 

 on the backs of the larger sea-weeds and other objects. 

 They look like a firm layer of hardish jelly, in which 

 are scattered "stars." We have several genera and 

 species of these pretty creatures, all of which are 

 apt to make their appearance in healthy tanks. 

 Some are joined at their bases by means of a thread- 



