38 INVERTEBRATE PHYSIOLOGY 



successful. In spite of the work of such deservedly famous investigators 

 as Romanes, Loeb, Pearl, Jennings, Parker, Koehler, and Pantin, we still 

 have very little of the basic physiological information that we need before 

 we can attempt any reasonably complete explanation. For instance, we 

 shall see that chemoreception plays an important role in these responses ; 

 yet the receptor units detecting these environmental clues have not often 

 been identified. But if any generalization can be made, it is that various 

 quite simple metazoans code their sensory input, so that relatively weak 

 stimulation of mechano- and chemoreceptors leads to food-capture activi- 

 ties, whereas strong stimulation leads to defensive reactions. Even in such 

 aganglionic animals as sea anemones, there is no single form of stimulation 

 that invariably leads to withdrawal or defensive responses. 



While many of the invertebrate phyla include species which lead an 

 active, carnivorous existence, hunting their food, it is often impossible to 

 obtain from the literature sufficient information both on their natural prey 

 and on the predators which, in turn, feed on them. One might imagine that 

 the success of certain forms with highly specialized diets, such as the 

 gastropod Archidoris which feeds on certain sponges, or many of the nudi- 

 branchs (Hunt, 1925) which feed on hydroids, is determined by the availa- 

 bility of their food rather than by predation. But certain of the latter group 

 clearly exhibit withdrawal behavior when they are vigorously prodded, 

 suggesting that their protective devices of noxious taste, slime, and hydroid 

 nematocysts may not always be sufficient deterrent to hungry fish or crabs. 



A number of the carnivorous mollusks, however — and I am thinking 

 here of such gastropods as Natica, Murex, Nassa, and Urosalpinx — are 

 known to constitute an important part of the diet of fish (Hunt, 1925; 

 Hancock, 1955) and starfish. Most of these gastropods can be distinguished 

 from their herbivorous cousins by the reduction or loss of the crystalline 

 style (Yonge, 1930). It is interesting to note that these snails are not 

 generally very restricted to certain prey ; Urosalpinx, for example, might 

 as fairly be termed the barnacle-drill or the mussel-drill as the oyster-drill. 



From the work of Copeland (1918) it seems quite certain that chemore- 

 ception is important in guiding these mollusks to their food. He showed 

 that in Nassa (= Alectrion) the sensory organ, the osphradium, samples 

 the water which enters the siphon. The animal responds to dilute food ex- 

 tracts by increasing its rate of locomotion, extending its proboscis, and 

 orienting the direction of movement towards the increasing chemical con- 

 centration. In addition to this "nasal" response, moreover, these snails 

 show "taste" responses to relatively strong food solutions with tentacles, 

 underpart of head, and anterior end of foot. There is no evidence that the 

 eyes play any part in food capture. 



It would be interesting to know whether or not tactile cues, in the ab- 



