Pelagic Fauna of our Shores. 139 



Pertdt'nium, and Ttntmnus, and they are also present in the 

 stomachs of annelids and other forms more or less fixed to 

 the bottom. The free-swimming larvee of the sponges, as 

 formerly indicated, also materially augment the pelagic fauna. 



Amongst ccelenterates the various true Medusse with their 

 larval forms are often in immense abundance, and so are the 

 Hydromedusee and their ciliated planulas. The most common 

 types of the Hydromedusse are Thaumantias, Bougainvillia, 

 Tivn'iSj Oceania^ and the little zooids of Ohelia^ a zoophyte 

 which forms a dense coating on the ropes of stake-nets for 

 salmon and other submerged structures with considerable rapi- 

 dity. In the same way the Zetlandic waters are occasionally 

 rendered phosphorescent by multitudes of similar free buds. 

 The ctenojihores [PleurohracMa and Beroe) are also often very 

 abundant. In connexion with the occurrence of the foregoing 

 ccelenterates, it is well to point out that they do not always 

 come to the surface, indeed they may be in vast multitudes 

 though not a single example is seen. As a rule, Medusse of 

 various kinds appear to pass their younger stages in the lower 

 regions of the water, and only come upwards in certain seasons 

 and under certain conditions. Their presence is often first dis- 

 covered by the trawl, and subsequently they are noticed on a 

 calm summer evening at the surface, which is broken here and 

 there by their contractions. The reasons for this irregularity 

 in the appearance of the Medusae are not at present clearly 

 understood, and they are not altogether due to atmospheric or 

 oceanic causes. 



The echinoderms, from holothurians to rosy feather-stars, 

 add largely to the temporarily pelagic fauna, since the larvsB 

 in most of the genera mount near the surface and only return 

 to the bottom after the adult shape is assumed. The influence 

 which even such forms as the Ophiuridse, which generally 

 live so buried in sand as to escape the trawl, have on the 

 pelagic fauna is considerable, since their larval Plutei swarm 

 near the surface. 



The larval annelids, again, are almost universally pelagic, 

 and off both sandy and rocky shores as well as everywhere on 

 the bottom a vast number are annually set free. Some adults, 

 again, as loida and Tomoptevis^ are only found in a pelagic 

 condition, and others assume it in special phases connected 

 with reproduction. Sagitta follows the same habit, and is 

 occasionally observed in enormous numbers, their bodies 

 sparkling like endless lines of glassy needles as they are 

 stranded by the tide on the beach. The larval annelids 

 affect the whole body of the water, and the same may be said 

 of Sagitta. 



