FROM THE EDDYSTONE GROUNDS TO START POINT. 445 



The distribution of such species depends upon that of suitable hosts, 

 and since the hosts are all wandering animals, the nature of the 

 bottom-deposit, when it is once sufficiently firm to support them, has 

 little direct effect on their distribution. 



d. Hydroids which generally fix themselves to stationary living 

 organisms or remains of organisms. We may to some extent distin- 

 guish between those which most frequently fix (1) to dead shells of 

 molluscs,* (2) to sea-weeds, (3) to other zoophytes, (4) to the mem- 

 branous or leathery tubes of polychaetes, (5) to living lauiellibranch 

 molluscs. Typical examples of (1) are Eudendrium rameum and 

 rariiosum, Campamdaria veriicillata (?), and Lafoea dumosa, var. rohusta; 

 of (2) Sertularia pumila and Ohclia geniculata ; of (3) Eudendrium 

 ca'pillare, Coppinia arcta, Halecium tenellum ; of (4) Ifalecium hale- 

 cinum, Sertidarella Gayi, Lar sabellarum ; of (5) Tuhiclava cornucopiae. 

 The distribution of these forms depends almost entirely upon the 

 distribution of their hosts, which are themselves fixed organisms 

 generally directly attached to the bottom, whose distribution therefore 

 is largely influenced by the nature of the bottom-deposit, 



Tuhiclava cornucopiae (Table VI., p. 529). This species was abundant 

 on the Bolt Head Shell Gravel (Ground XVII). In haul 105, in which 

 Pectuncidus glycimeris was abundant, nearly every living specimen 

 of that mollusc had Tuhiclava growing in considerable quantity on 

 the posterior margin of the shell. In haul 107 on the same ground 

 Tuhiclava cornucopiae was growing on the posterior end of living 

 Venus fasciata. 



The hydroid was also present on the fine sand south of the 

 Eddystone (Ground VI., haul 102), growing on the posterior end of 

 the shell of Dentalium entalis. 



DiSTKiBUTiON. Geographical. Shetland (Norman, No. 90) ; Northumberland (Alder) ; 

 Plymouth (Garstang, No. 27, p. 334 ; No. 28, p. 212). 



Depth. 15 fathoms (Garstang) to 80-100 fathoms (Norman). 



Habitat. Norman found his specimens growing on Astartc sulcata and Dentalium 

 entalis. In every instance the mollusc was living and the hydroid was on the posterior 

 extremity of the shell, where, as Norman says, " it would receive the henefit of the 

 aqueons currents caused by the mollusc, which, while 2>ioviding for its own necessities, 

 thus unwittingly performed the kindly office of feeding its hungry neighbour." Alder's 

 specimens were also found on the posterior end of Dentalium entalis. 



Garstang found the species ujmn Aporrhais pcs-pelecani shells inhabited by Flmscolion 

 strombi, and subsequently also on shells of Turritella inhabited by the same Gepliyrean. 

 These specimens were obtained in 1.5-25 fathoms, south of the Plymouth Mewstone. The 

 species is still frequently taken in this locality on a bottom of clean shell gravel. 



Speaking of Pectunculus glycimeris, Gwyn Jeffreys (No. 55, p. 168) says, "The anterior 

 side of the sliell, while the animal is alive, is frequently fringed with the tubes of a 



* Those hydroids which fix on shells are less clearly marked as a separate class, proba})ly 

 since a dead shell offers conditions very similar both to those of stones and of other living 

 organisms. 



NEW SKRIES. — VOL. V. NO. 4. ■^ Q 



