Chap. I. STRUCTURAL FEATURES. 171 



animals, which they digest very rapidly. I have seen an Idyia swallow a Bolina 

 nearly as large as itself and consume it in half an hour, rejecting the more con- 

 sistent parts, such as the locomotive flappers, etc. During the process of digestion 

 the funnel is greatly distended, and the chymiferous tubes swell enormously, so 

 that their diameter much exceeds the width of the rows of locomotive flappers, 

 and an active circulation may be traced through the wliole system, when even 

 the most minute branches ajipear gorged with fluid. Undoubtedly this fluid is 

 subservient to the purpose of alimentation; and the whole body enlarges, and, when 

 full-grown, derives its maintenance, through what it assimilates out of it. But 

 when considering its relations to the whole organism of the Ctenophorte and the 

 part it plays in their general economy, it should not be overlooked that it is 

 neither blood, nor in any way comparable to the fluid circulating in the vascular 

 system of either the Vertebrates, the Articulates, or the Mollusks, but simply a 

 fluid elaborated in the digestive cavity by a disintegration of the food, with Avhich 

 a large quantity of water is mixed, and then, after the coarser parts have been 

 rejected through the mouth, directly poured into another cavity, the funnel, whence 

 it passes into the radiating tubes, parts of it being froni time to time discharged 

 through the two openings near the abactinal pole. 



In consequence of these very peculiar combinations, it is not easy to ascertain 

 what these openings are. Some anatomists vmhesitatingly call them anal apertures ; 

 but the anus, throughout the animal kingdom, is the posterior opening of the 

 alimentary canal, while here we have two openings in the central part of a system 

 which is distinct from the digestive cavity, and through which nutritive fluid is 

 circulated. These openings cannot, therefore, be identified with anal apertures, even 

 though the nutritive mass he only chyme, and neither chyle nor blood ; but ought 

 to be considered as a special structural combination i^eculiar to this type of the 

 animal kingdom. It cannot be strictly homologous to any structure among the 

 higher types of animals ; and the nearest resemblance which it has to the structural 

 complications of the Vertel)rates is a mere analogy, and may be thus expressed : 

 Supposing the alimentary canal and the lymphatic vessels arising from its wall to 

 be in direct and open communication with the veins which carry the products of 

 digestion to the heart, and through this with the arteries providing the materials 

 that are discharged by ^perspiration at the surflice of the body, and that the fluids 

 moving through this succession of distinct structural systems should be throughout 

 the same fluid, and the channels through which it is discharged at the surface, 

 irot oirly unobstructed by intervening capillary networks, Imt uniform from one end 

 to the other, and we should have homological structures. The difference between 

 the one and the other case shows the remoteness of their analogy ; and, instead of 

 calling these openings (iiial apertures, I would prefer the name of ca'liac apertures. 



