REPRODUCTION AND GROWTH 249 



{Asterina, Murex, Blennins^ etc.), fix their eggs securely 

 to the rocks and protect them by a capsule. The shallow 

 and stagnant waters of harbours, which are subject to intense 

 putrefaction and excessive heating, especially in summer, 

 are populated by species which mostly reproduce in winter 

 and spring ; such commensal and parasitic species as live 

 in an unvarying medium lay their eggs at every season. 

 Certain Hydroids {Pennaria, Corydendrium) shed their 

 polyps and enter into a state of latent life when the autumn 

 storms come on, giving off new branches and developing 

 fresh polyps in the following May. Lo Bianco interprets 

 this as a protection against wave action, but Issel (1918) 

 thinks other factors : temperature, density, food, are 

 concerned as well. 



Of the several species of Littorina which are found on 

 the shore L. littorea^ which occurs nearest to low- water 

 mark, deposits its ova in small capsules, shaped like a soldier's 

 tin hat. They are not attached. The embryo is freed as an 

 early veliger. But the larvae of L. obtusata, which occurs at 

 about half-tide mark, are freed as second stage veligers, while 

 L. rudis and L. neritoides, the latter of which lives actually 

 above high- water mark, are viviparous (Tattersall, 1909). 

 The importance of this is clear. As one proceeds up the 

 shore the periods of drought become longer and the risks to 

 a free-swimming larva considerably greater. Consequently, 

 there is a tendency to abbreviate the free-swimming stage 

 and even to cut it out altogether. Again, the Nudibranch 

 Centa, which frequents brackish estuaries and even possesses 

 to a limited extent the habit of leaving the water, has an 

 abnormal development which takes place within the eggs 

 (Eliot, 1910). 



An analogous case is that of the viviparous blenny 

 {Zoarces viviparus) whose habits are in marked contrast to 

 those of other shore-frequenting fish. Instead of depositing 

 demersal eggs in spring or summer, like most shore fishes, 

 this species of blenny produces well- developed living young 

 in winter (Aflalo, 1904). According to M'Intosh (1901), an 

 adult female may produce from 40 to 70 living young capable 



