24 MARINE ANIMALS OF MASSACHUSETTS BAY. 



and dies, and the cycle is complete. The autumn storms make 

 fearful havoc among them, swarms of them being killed by the fall 

 rains, after which they may be found thrown up on the beaches 

 in great numbers. When we consider the size of these Jelly- 

 fishes, their rapidity of growth seems very remarkable. Our 

 common Aurelia measures some twelve to eighteen inches in 

 diameter when full grown, and yet in the winter it is a Hydra so 

 small as almost to escape notice. Still more striking is the rapid 

 increase of our Cyanea, that giant among Jelly-fishes, which, 

 were it not for the soft, gelatinous consistency of its body, would 

 be one of the most formidable among our marine animals. 



Before entering upon the descriptions of the special kinds of 

 Jelly-fishes, we would remind our readers that the radiate plan of 

 structure is reproduced in this class of animals as distinctly as in 

 the Polyps, though under a different aspect. Here also we find 

 that there is a central digestive cavity from which all the radiat- 

 ing cavities, whether simple or ramified, diverge toward the peri- 

 phery. It is true that the open chambers of the Polyps are here 

 transformed into narrow tubes, by the thickening of the dividing 

 partitions, or in other words, the open spaces of the Polyps cor- 

 respond to tubes in the Acalephs, while the partitions in the 

 Polyps correspond to the thick masses of the body dividing the 

 tubes in the Acalephs ; but the principle of radiation on which 

 the whole branch of Radiates is constructed controls the organi- 

 zation of Acalephs no less than that of the other classes, so that 

 a transverse section across any Polyp (Fig. 1), or across any 

 Acaleph (Fig. 50), or across any Echinoderm (Fig. 140), shows 

 their internal structure to be based upon a radiation of all parts 

 from the centre to the periphery. 



That there may be no vagueness as to the terms used here- 

 after, we would add one word respecting the nomenclature of this 

 class, whose aliases might baffle the sagacity of a police detective. 

 The names Acalephs, Medusae, or the more common appellation 

 of Jelly-fishes, cover the same ground, and are applied indiscrim- 

 inately to the animals they represent. The name Jelly-fish is an 

 inappropriate one, though the gelatinous consistency of these 

 animals is accurately enough expressed by it ; but they have no 

 more structural relation to a fish than to a bird or an insect. 



