376 THE BIOLOGY OF MARINE ANIMALS 



head attached to the central core of the tentacle by coiled contractile and 

 straight filaments. Within the head are granules which discharge a sticky 

 secretion used in capturing prey (56). 



CONTRACTILE CELLS IN SPONGES 



Although sponges lack a nervous system they are capable of simple 

 contractile responses. When a sponge is exposed to air, or lies in still 

 water, the flesh contracts and takes on a wrinkled appearance (Stylotella). 

 On returning the animal to running water the surface expands once more. 



Fig. 9.5. Transverse Section through a Young Cnidophore Sac of a 

 Nudibranch (Aeolidiella glauca), Containing Nematocysts of the Anemone 

 Cereus pedunculatus (from Naville, 1926.) 



The most evident activity of sponges, however, is the production of a 

 feeding current by the continuous beating of choanocytes. The feeding 

 current is interrupted under unfavourable conditions, e.g. in still or 

 deoxygenated water, in the presence of noxious chemicals, etc. A current 

 can flow only when the dermal pores and the oscula are open; when these 

 are shut the current stops. The elements responsible for contraction are the 

 porocytes surrounding the pores, and certain epithelial myocytes lining 

 the pore canals and oscula. These cells behave as independent effectors, 

 i.e. they respond directly to an external stimulus by contraction. The re- 

 sponse is very slow, an osculum of Stylotella closing in about 3 min after an 

 external stimulus such as a pinprick. Each pore and each osculum responds 

 independently of the others, but when the stimulus is widespread the sum 

 effect of the contractions of all the porocytes and myocytes is a general 



