248 THE BIOLOGY OF THE SEA-SHORE 



pelagic larvae is not so fortuitous a matter as one might 

 suppose . The risks , it is true , in spite of adaptive phenomena 

 are still formidable, but against this we may place the 

 fecundity of such forms and the fact that the larvae are 

 usually hatched at the season of calms. 



Places selected for Spawning. — The risk of desiccation 

 is countered, in the case of eggs fixed to the substratum 

 within the tidal area, by the fact that such eggs are usually 

 surrounded by a protective gelatinous envelope or, in the 

 case of embryos, by a horny capsule. Fronds of algae are 

 favourite places for the deposit of such spawn, the weed 

 lending its moisture and protecting the eggs from evaporation. 

 Forms in which fertiUsation is external {e.g. most Annelids) 

 may liberate their reproductive products in shore pools at 

 low tide. Where the spawn is deposited on the substratum 

 the place selected is almost invariably some sheltered cranny 

 or nook formed at the junction of rocks, by a fault in the 

 rock itself or by an overhanging ledge. Often the spawn is 

 so abundant as to suggest the term " nursery " for such 

 nooks. 



Abundant use is made of the protection for the ova and 

 developing young offered by the empty shells of molluscs, 

 the empty cups of barnacles, etc. Empty barnacle shells 

 frequently contain numerous tiny Littorinas. The larvae 

 of Mytilus edulis may invade the empty air vesicles of 

 Ascophyllum, occasioning a protective reaction of the plant 

 (Tobler, 1909). The roots of Laminaria afford similar shelter 

 for tiny mussels and numerous other young forms. 



Adjustment of Life Cycle. — In view of the many vicissi- 

 tudes of shore life and the variable character of this environ- 

 ment generally, it is not surprising to find on the shore 

 numerous adjustments of the individual life cycle. For 

 instance, Lo Bianco (1911) finds that the species in the Gulf 

 of Naples which live continuously exposed to wave-shock 

 spawn almost invariably at the season of calms (or during 

 the warmest months of the year), or otherwise the larvas 

 would not survive. The small minority which form an 

 exception to this rule, spawning in winter and spring 



