SPONGES, ZOOPHYTES, AND SEA-FIRS. 55 



distinguish at a glance the species of Sertularia, which have 

 usually opposite cups, and stems which appear to be made 

 up of a succession of triangular joints, the base of the 

 triangle being directed upwards. By far the commonest 

 species is S. pumila, an insignificant little zoophyte, which, 

 with its loosely branching stems, often occurs in great pro- 

 fusion on the shore rocks. It has a special preference for 

 the blades of the larger weeds, and is readily recognised by 

 the regular V-shape of the joints of which the stem is 

 composed. The cups in which the zooids are placed form 



FIG. 20. Sertularia pumila, and a magnified representa- 

 tion of a portion of a branch, a, gonotheca ; &, empty 

 zooid-cup. After Hincks. 



the upper part of the diverging arms of the V (see Fig. 20). 

 We may repeat here that to recognise the species of sea-firs 

 requires a little skill and the use of the microscope. The ex- 

 amples which have been briefly described are intended to 

 give the student some notion of the modifications of structure 

 seen in the chief families, and assist in the recognition of at 

 least the family of the common forms. More than this will 

 probably be found difficult for the beginner. Several species 

 of Sertularia are fairly common between tide-marks, and 

 others are frequently thrown up on the beach, and are to 

 be found attached to other animals so thrown up. To settle 

 the species of these is often difficult, but it is much to learn 

 that they are Sertularians, and to realise their differences 

 from the other hydroids often so plentiful in the same place. 

 Before we leave the family two other forms may be 

 briefly mentioned, which differ very much in appearance 



