PLANKTONIC LARVAE 



The planktonic incdusoid stage of the attached hydroids has ah'cady 

 been mentioned (p. 48), but although the medusa is the adult sexual stage 

 it is the attached hydroids on the sea shore that will be most familiar to 

 the readers of this book and so the medusae deserve this passing mention 

 again in this chapter (Figs. 13 and 14). Most sea anemones attached to 

 the rocks have a very short planktonic stage — the planula — \\4iich soon 

 settles down, but one group, the family Cerianthidae, live offshore on the 

 bottom usually in nniddy sand, and their larvae have a prolonged planktonic 

 stage called an arachnactis larva (Fig. 17; 8). These were at one time thought 

 to be separate species of animals and had their own names such as AracliinKtis 

 hoiiriiei and A. cilhidci. They are especially common in warm oceanic water, 

 but are also found in inshore areas. 



Each phylum of that miscellaneous group called 'worms' has its own 

 characteristic type of larva but many species have not as yet been associated 

 vv^ith their larvae. The nemertines — proboscis worms — have a pilidium 

 larva (Fig. 24; i) but the nematodes — round worms — have no planktonic 

 stage. The larvae of the bristle-worms or Polychaeta are called trocho- 

 phores which after a while begin to segment (Fig. 25). These larvae appear 

 to have the ability to prolong their planktonic life considerably if the con- 

 ditions (e.g. the texture of the bottom) are not suitable for their metamor- 

 phosis, and indeed recent work at the Plymouth laboratory indicates that 

 their requirements are very tniicky indeed. Ophelia biconiis larvae, for ex- 

 ample, will only settle on clean loose sand of the right size and consistency. 

 If the grains are too small the larvae cannot wriggle into the spaces, if the 

 sand is not clean enough the spaces get choked with silt or tmer particles, 

 and yet there must also be enough food for them to live on. The sand must 

 be roughly rounded and not sharply pointed; sand lying loosely in places 

 where there are strong tidal currents has the right mixture of characters. 

 A series of drawings of some of the types of polychaete larvae is given in 

 Fig. 25 and photographs in Plates XXII and XXIII. 



Other 'worms' which have characteristic larvae are Plioroiiis with its 

 'actinotrocha' (Fig. 24; 2 and Plate XXIII) and the sea mosses and sea mats, 

 or Bryozoa with their 'cyphonautes' (Fig. 24; 3) which can often form an 

 important fraction of the plankton in the spring before they settle down on 

 the sea floor (sea mats — Fliistra) or to encrust the seaweeds and rocks (sea 

 mosses) . 



8-12. Lainellibranchiata (bivalve imilkiscs): 



8. TellithJ sp. 



9. Macoma halticn. 



10. Ccmiimu echile (cockle). 



11. Mytiliis eduUs (mussel). 



12. Pccten opercularis (queen, or small scallop). 



[1-7 from Thorson, 8-12 from J<^rgeiisen.] 



85 



