the Convergence of Types by Pelagic Life. 85 



is combined with the permanent fixation and complete depen- 

 dence of the parasite with relation to the affected organism. 

 Parasitism in this sense produces results such that the zoolo- 

 gist, furnished only with the resources of anatomy and pure 

 morphology, could never have referred certain animals to their 

 true place in the classification. 



But there are other groups of etiological conditions 

 which, without acting in so remarkable a manner, neverthe- 

 less induce very interesting typical convergences, especially 

 when they affect simple or feebly differentiated organisms. We 

 shall pay attention at present only to pelagic life, and seek to 

 determine with precision what are the modifications that this 

 mode of existence may induce in the animals of various classes 

 that are subjected to it. This will enable us to appreciate the 

 value of the reasons which have led zoologists to place Sagitta 

 in one group or another of the animal kingdom. 



Pelagic animals are those which live in the open sea, 

 generally near the surface of the water, and rarely approach 

 the shore, upon which, however, they are sometimes cast by 

 the winds. We find animals leading such an existence 

 throughout the whole zoological series, from the Protozoa to 

 the Vertebrata. If we leave out of consideration superficial 

 currents and climatal zones, these animals live under very 

 uniform and at the same time very special conditions, the 

 action of which must impress upon the organism certain pecu- 

 liar features, which may succeed in masking the morphological 

 type, especially in the Invertebrata. 



The characters of adaptation proper to pelagic life are : — 



1. An extreme transparency of all the tissues, which ren- 

 ders the animal completely invisible, and enables it to escape 

 easily from its enemies. This transparency exists in animals 

 belonging to the most diverse groups. We observe it in the 

 Noctilucce, the Siphonophora, the Medusas, the Ctenophora, 

 the Heteropod and Pteropod Mollusca, the Saljxe and Pyro- 

 somata) in Sagitta, Tomopteris, and Alciope' and, lastly, in 

 the Leptocephali among fishes. 



2. The considerable development of certain organs of the 

 senses, which often constitute the sole visible points of the 

 animal. In general it is the eyes that present an enormous 

 development with relation to the rest of the organization, as 

 may be observed in a great number of the examples just 

 cited ; sometimes also the auditory apparatus, as in the 

 Medusae and the Appendicular ice, and in Mysis, in which this 

 apparatus is situated upon the caudal lamina?. 



3. A reduction of the digestive tube, which becomes consi- 

 derable, although without being so marked as in parasitic 



