60 LIFE BY THE SEASHORE. 



to believe that the condition of each is an adaptation to its 

 own particular surroundings, yet we are unable to say how 

 the surroundings differ, or wherein the adaptation consists. 

 Nor can we say that the difference is associated with some 

 other structural peculiarity, for as yet it is not possible to 

 point out any constant difference between those zoophytes 

 which produce medusoids and those producing sporosacs, 

 apart from this prime difference. It is the constant occur- 

 rence of phenomena like this which makes shore life so 

 interesting, and its study so helpful to those especially 

 whose scientific training has been largely that of the 

 laboratory. 



Though we cannot tell whether a hydroid colony will 

 produce sporosacs or medusoids, apart from the actual 

 experience which shows us what it does produce, yet it is 

 interesting to note that there is a permanent structural 

 distinction between the swimming-bells of the Calypto- 

 blastea and those of the Gymnoblastea, so that we can 

 determine the nature of the colony from which any par- 

 ticular swimming-bell has arisen. We cannot here describe 

 in detail these differences, but may note that in the former 

 case the reproductive elements are produced in the rnanu- 

 brium, or clapper of the bell (cf. Sarsia), while in the latter 

 they arise in the course of those radial canals of which 

 mention has been made (p. 47). Both kinds of medusoids 

 are common in our seas, but in most localities those of the 

 species of Obelia are perhaps commonest of all. They may 

 be recognised by their peculiarly flattened shape, and the 

 short distinctly four-lipped manubrium. The tentacles are 

 short and numerous, and the four sets of reproductive 

 organs are very distinct. The size varies from about that of 

 a sixpence to that of half a crown, and the creatures resemble 

 transparent plates rather than bells. Another very common 

 swimming-bell, that of Clytia johnstoni, is shown in Fig. 90, 

 p. 326. It differs chiefly from that of Obelia in having only 

 four tentacles. 



We shall conclude this chapter by giving a table which 

 may assist the student in the identification of the common 

 sea-firs. 



