108 LOWER INVERTEBRATES. 



which is commonly submerged as the animal floats on the surface of the water. Of 

 these perhaps the most prominent is a centrally placed body which hangs downward 

 below the remaining appendages, and is open at its unattached extremity. This 

 structure is the feeding-mouth or polypite, and is single in both Velella and Porpita. 

 In the zone just surrounding the polypite we find a large number of small appendages, 

 each of which has a thread-like shape bearing along its sides a number of little trans- 

 parent buds in all conditions of growth. Each of these little bodies ultimately sepa- 

 rates from its attachment, and in the form of a minute jelly-fish, not larger than a pin- 

 head, swims about endowed with independent powers of life for a considerable length 

 of time. The medusa which has thus separated is known as a Chrysomitra. Surround- 

 ing the bodies last mentioned on the lower surface of Velella, there is a circle of 

 feelers of bluish color which are commonly in constant motion. One of the most 

 prominent superficial differences between Porpita and Yelella is the total absence of 

 a triangular sail in the former genus. They are commonly found associated together, 

 and often accompanied by a third jelly-fish allied to both, called Rataria. 



Class IY. — CTENOPHOKA. 



The highest of the jelly-fishes, both on embryological and anatomical grounds, are 

 known as the Ctenophora. In these animals that characteristic of higher forms of life 

 known as bilateral symmetry appears for the first time. An obscure symmetry which 

 has been called by some naturalists bilateral, aj)pears among the Siphonophora, and 

 even in the Hydroidea. In the Ctenophora, however, it is more plainly indicated than 

 in either of these groups. 



One of the earliest mentions which we have of the ctenophorous medusae we owe 

 to Martens. Freiderich Martens was a ship's pliysician or a ship's barber, as he styles 

 himself, who accompanied Captain Scoresby in a voyage of exploration into the Polar 

 Seas. He first found one of these beautiful medusai in the neighborhood of the island 

 of Spitzbergen. Eschscholtz was the first to recognize the common likenesses between 

 the different members of the group, and gave to them the name of Ctenophora or 

 comb-bearing medusae. 



The Ctenophora take their name, as he first pointed out, from the existence on the 

 external body walls of eight rows of vibratile plates called combs. These combs are 

 arranged in such a way that in flapping they strike upon the water, and by their mo- 

 tion the jelly-fish is driven along through the water. We find here for the first time 

 since our studies of the Calenterata began, a large group of animals where movement 

 in the water is produced both by special locomotive organs and contractions of the 

 body. 



The varieties in form in the bodies of different Ctenophora is very great. In some 

 genera they appear as long ribbon or belt-like creatures which move through the water 

 with serpentine movements, in others as transparent caps or globular gelatinous masses 

 over which the rows of combs shine with most lovely iridescent colors. No greater 

 variety of more beautiful genera is to be found anywhere among the medusae. 



Cestics, called also the Venus Girdle, is perhaps the most striking genus of the 

 Ctenophora. Its shape departs the most widely of all the Ctenophora from that of the 

 medusoid types. In Cestus the body of the jelly-fish has a girdle or belt-like form, and 

 is moved more by the contractions of the body than by the rows of combs which fringe 

 its edges. The animal is very transparent and extremely tender, so that it is with the 



