86 M. A. Giard on the Position ofSagitta, and on 



animals. Not to mention the numerous examples of this 

 reduction that may be found among the Medusas and Cteno- 

 phora, we meet in other groups with an atrophy of the diges- 

 tive organs which may even reach complete disappearance 

 absolutely, as in the Rhizostomes. This is what occurs, for 

 example, in the curious genus Monstrilla, one of the pelagic 

 Copepod Crustacea ; the nucleus of the Salpce and Appen- 

 dicularice also represents a perfectly rudimentary state of the 

 digestive tube of the Tunicata, if it be compared with the 

 intestinal mass of the animals of that group which lead a 

 sedentary life (Ascidia). The same may be said of the 

 digestive tube of Carinaria, Firoloides, Atlanta, &c, when 

 compared with that of the ordinary Gasteropoda. Finally, 

 the Sagittal also present an excessively reduced digestive 

 tube, which occupies only a small portion of the length of the 

 body. 



This reduction of the digestive system in pelagic animals 

 is evidently in relation with the precarious existence of these 

 creatures, which are constantly pursued by numerous enemies. 

 A voluminous stomach would impede their progress, which 

 is generally very rapid, and would diminish the transparency 

 which protects them. 



4. A considerable development of the organs of generation 

 and great fecundity. Here again it is sufficient, in order to 

 ascertain this fact, to compare in the same group the pelagic 

 animals with those which live attached. In Append icidaria, 

 for example, the genital mass is much more voluminous than 

 in the Ascidia, taking the proportion of this mass to the total 

 volume of the body of the animal. This excessive multiplica- 

 tion of the pelagic animals must be attributed to the numerous 

 chances of destruction to which creatures so badly protected 

 are exposed, just as in the case of the parasites, among which 

 the same fact is also observed. 



5. A great number of pelagic animals present the pheno- 

 menon of phosphorescence, such as the Noctilucw, many 

 Medusas, the Pyrosomata, and Phyllirhoe bucephala. This 

 phosphorescence, which is manifested especially when the 

 animals are excited or alarmed, no doubt acts as a protection, 

 and stops the pursuit of some enemies*. I have not remarked 

 that the Sagittal are endowed with any such means of defence, 

 which, moreover, is far from being peculiar to pelagic 

 animals. 



6. As an ethological character frequently observable in 

 pelagic animals, we must cite social life : we know what 



* Panceri and De Quatrefages have made the very interesting observa- 

 tion that phosphorescence is under the control of the nervous system. 



