238 THE BIOLOGY OF MARINE ANIMALS 



mechanical and chemical stimuli. Owing to rapid adaptation mechanical 

 stimuli rarely induce a complete response, but the intervention of proper 

 chemical stimuli usually results in acceptance of foodstuffs. Of a range of 

 chemical substances tested, the most active are proteins and their deriva- 

 tives, including peptones and various amino-acids. Certain lipoid extracts 

 are also effective, but not carbohydrates. This selective sensitivity is 

 obviously closely related to the purely carnivorous habits of these 

 animals (85). 



All ctenophores are carnivorous in feeding habits. Tentaculate forms, 

 exemplified by Pleurobrachia and young Mnemiopsis, capture small 

 plankton organisms with their tentacles. These are provided with sticky 

 lasso cells known as colloblasts, which hold on to the food. After making a 

 successful capture the tentacle contracts and conveys the food to the mouth. 

 In adult Mnemiopsis a complex ciliary and tentacular mechanism is em- 

 ployed. Extending along the sides of this animal are four auricular grooves 

 into which food particles are carried by beating cilia. At the bottom of a 

 groove the particles become entangled on small tentacles : these bend over 

 into a labial trough and food particles are conveyed down the latter channel 

 to the mouth. Non-tentacular beroids capture their prey by means of the 

 extensible mouth rim and can ingest relatively large animals such as 

 crustaceans and other ctenophores (48, 67). 



Turbellaria. These animals possess a muscular pharynx, which can be 

 protruded for capturing food. This consists of a variety of small animals — 

 protozoa, nematodes and small crustaceans. Cycloporus (a polyclad) feeds 

 on colonial tunicates, and sucks out zooids individually (50a). 



Nemertines are entirely carnivorous when adult, feeding on a variety of 

 prey. Immature and small animals feed on protozoa. Larger benthic and 

 littoral species capture small Crustacea, worms, molluscs and even small 

 fish, living or dead; pelagic nemertines subsist on small Crustacea. Food 

 is captured with the aid of a muscular proboscis which can be everted for 

 some distance in front of the head, and in some species it is actually as 

 long as or longer than the animal's body. When the proboscis is shot 

 forth it entwines itself around the prey, which is retained by tenacious 

 mucus or quietened by means of immobilizing secretions. Moreover in 

 some species the proboscis is armed with sharply pointed stylets (Hop- 

 lonemertea, e.g. Amphiporus). The food is then conveyed to the mouth to 

 be swallowed entire. A large Cerebratulus, for example, can swallow an 

 annelid nearly equal to its own diameter. An aberrant form, Malacob- 

 della, is commensal in the mantle cavity of bivalves (Seliqua), where it 

 feeds on plankton filtered off by the host (16). 



Annelids. Many polychaetes have muscular introverts armed with small 

 teeth, e.g. Aphrodite, Lumbriconereis, Nephthys, Glycera, etc. The introvert 

 is used for capturing prey, which consists usually of living animals such as 

 worms, molluscs and small crustaceans. Nereis virens, an errant carnivore, 

 also feeds on dead animals and algae. Tomopteris, a voracious planktonic 

 form, swallows entire Sagitta and larval herrings. Certain syllids, e.g. 



