OxtveR.— Marine Littoral Plant and Animal Communities. 515 
tectibranchs are sometimes of mottled greys like Pleurobranchus novae- 
zealandiae, or reddish-brown with darker blotches as in P. ornatus—that 
is, they resemble the surroundings generally. Others are so coloured as 
to resemble only the algae or sponge on which they are found. Thus 
Ctenodoris flabellifera is yellow, and Rostanga rubicunda bright scarlet, 
and these are found on sponges similarly coloured. The free sea-anemone 
` Phlyctenactis retifera is coloured olive-brown like Cystophora dumosa, on 
which it lives. Crustacea are green (Paridotea ungulata), brown (Amphoroidea 
paused for a moment, then hurried back to the seaweed-covered rocks. 
Many animals have their shells or tests (Tunicata) masked by algae, 
Vermilia, barnacles, sponges, and other forms of life. These probably 
merely settle uninvited on the vacant space. Nevertheless, they must 
Subserve to some extent the purpose of concealment. Mud- and sand- 
growth. Too much growth on the shells sometimes results in the destruc- 
tion of both the mollusc and the life it supports. In the Bay of Plenty, 
after a storm, it is a common thing to find Venericardia purpurata with the 
sponge Chalina 30 cm. or more in length washed up on the beach. _ 
(6.) Animals and Plants.—Some animals appear to be specially modified 
for attachment to algae. When Siphonaria australis lives within the base 
of the holdfast of Durvillea antarctica it is usually found on the roof of 
the chamber, occupying hollows in the algal tissue. Here the foot of the 
mollusc fills up the hollow and projects beyond the shell-margin, the shell 
itself becomes depressed sometimes to the point of becoming quite flat, and 
the margin is slightly recurved. {с 
Notoacmea scapha is found in Tauranga Harbour living on the leaves 
of Zostera nana. Its narrow shell seems a special adaptation to this station. 
The hooks of the isopods Idotea and Amphoroidea for holding on to algae 
have already been mentioned. 
III. BIOTIC COMMUNITIES. 
Animals and plants inhabiting the littoral belt are found to group them- 
selves in communities, man wW. 
mmun 
imits assigned to them, whether from a consideration of the species they 
include or of the habitats, it is found that in every case both plants and 
animals form integral parts. The term “ growth-form ж 
as including all those species which agree in their external form, anatomica 
structure, and behaviour. A biotic community consists of a definite set of 
growth-forms in relation to a definite habitat. The growth-forms occur 
17* 
