112 LOWER INVERTEBRATES. 



It is by the strokes of these bodies on the surrounding water that the jelly-fish is 

 moved about from place to place. 



The tentacles of Pleicrobrachia, oftentimes almost wholly inconspicuous they are so 

 securely packed in little lateral pockets on either hemisphere of the medusa, are most 

 important structures in the economies of this beautiful medusa. It often happens, 

 when the jelly-fish is at rest, that the tentacles are extended from their pockets, 

 stretching far outside of this receptacle. Their length when extended in this way is 

 so great that it seems impossible that they can be retracted into the tentacular pockets. 

 The tentacles are two in number, each tentacle bearing a large number of side branches 

 of brownish color, and in the movements of the medusa are often thrown into the most 

 fantastic shapes. ■ 



The Ctenophora Nuda, or tho'se ctenojDhores which are destitute of tentacles, are 

 represented by a very beautiful medusa called Hero'e. This is perhaps one of the most 

 remarkable jelly-fish which we have yet considered, although it is without doubt one 

 of the lowest in its organization. The form of the body of this animal is that of a cap 

 or rounded sac of very simple structure. 



If we closely consider the structure of this animal it will be noticed that, extending 

 longitudinally across the outside surface of the body, there are eight rows of combs as 

 in the other Ctenophora. At the pole of the body there is a sense-body in a similar 

 position to that of the sense-body of other genera. The whole interior of the body 

 serves as a stomach, and into it there opens a mouth of very great size. The stomach 

 will often be found gorged with food, and the animal swollen to double its natural 

 size by the mass of food collected in this organ. This animal is indeed one of the most 

 ravenous genera of medusae. There are no tentacles in the genus Hero'e and no sign 

 of tentacular sacs. The chymiferous tubes have a very simple course in the walls of 

 the body, extending from common origin to the vicinity of the mouth in an almost 

 direct course with, however, many side branches. The color of our common Beroe is 

 a delicate pink. 



Class II. — ACTmOZO A. 



Much discussion, happily now for the most part of the past, hangs about our knowl- 

 edge of the nature of the Actinozoa or corals. Their relationship to animals was first 

 recognized by Peysonelle, who records his observations in the quaint language of a 

 former century. The word Actinozoa, of comparatively modern date, has an almost 

 €xact equivalent in the older term Zoophyte, and refers to a great variety of animals 

 fixed to the ground like plants, and possessing in common with them certain superficial 

 resemblances. They all have remote likenesses to a plant or flower, with which for a 

 long time in the history of science they were confounded. It includes the great families 

 of reef-building corals, and has an added interest on account of the many questions which 

 suggest themselves in relation to the method of formation of coral reefs and coral islands. 

 In the general structure of their bodies the Actinozoa differ somewhat from the Hydrozoa, 

 but the difference is not of such great importance as to call for a wide separation in a 

 scheme of classification of the two. The differences would appear at first sight very 

 great. Nothing for instance could seem more different than the soft, gelatinous body of 

 the medusa and the stony mass of a coral, and yet this difference has no homological 

 meaning, and as far as general structure is concerned the two are identical. In the 

 medusae the body is so filmy and transparent that it is often wholly invisible as the 



