44 



POLYPIFERA. 



longitudinal prominent bands, the free edge 

 slightly enlarged and rounded, a pedicle in 

 the middle like the stem of the vase, and the 

 transparent vesicle lining its interior through- 

 out. 



Fig. 49. 



A scries illustrating the development of Tubidaria by 

 free gemma-, from the first indication of the bud to 

 the time when it becomes detached. 



1. A hollow tubercle or elevation, in the interior 

 of which a movement or circulation of the glo- 

 bules, indicated by the arrow, takes place; a, a 

 cell just beneath the surface. 



2. The same, showing the cell more advanced ; a, 

 indicates this cell in all the figures. 



3. This, and the following figures, represent the 

 development of the gemma more and more ad- 

 vanced; b, a tubercle, situated beneath the cell, 

 which becomes the stomach of the embryo ; this 

 organ is indicated by the same letter in the other 

 figures. 



4. c, tubercles shooting up from the sides ; they 

 are hollow, and communicate with the cavity of the 

 stomach, and are the first indication of four vessels 

 proceeding from the stomachal cavity. In the fol- 

 lowing figures the letter c indicates these vessels. 



5. b, the tubercle become more elevated, indenting 

 the cell a ; the four secondary tubercles, c, more 

 distinct and prominent. 



6. The stomachal and its four surrounding hollow 

 tubercles still further prolonged. 



7. In this figure the four smaller tubercles have 

 become vessels, and united with one another in front, 



8. The four vessels have more completely united 

 in front ; the arrows here, as in the other figures, in* 

 dicate the current of the circulating fluid. 



9. d, The first indications of the tentacles, con- 

 sisting of tubercles sprouting from the four vessels. 



10. The tubercles, d, at the end of the four ves- 

 sels, c, have become sufficiently elevated to make 

 a projection on the exterior. 



11. These tubercles, become considerably salient 

 externally, are now manifestly the four tentacles of 

 the embryo. 



12. Minute cells are now visible at the extremity 

 of the tentacles. 



13. The tentacles still more advanced ; the line 

 of separation of the embryo from its stalk become 

 distinctly visible. 



14. Hitherto the stomachal cul-de-sac has pro- 

 gressively increased, it now begins to diminish, and 

 the cell a, or the space between it and the external 

 envelope becomes opened at e, forming a kind of 

 mouth ; the embryo is now capable of great exten- 

 sion ; the pedicle is constricted at the point of 

 insertion and its internal cavity nearly obliterated. 

 {From Van Beneden.} 



The different phases of the development 

 above described will, however, be best under- 

 stood by a reference to the series of figures 

 which we have appended, carefully copied 

 from Professor Van Beneden's elaborate illus- 

 trations. 



The }oung Tubularia has now assumed the 

 appearance of a Beroe, and in this condition 

 has doubtless been often mistaken for an in- 

 dividual belonging to the class Acalephae ; and 

 lively contractions of its body are frequently 

 witnessed, although it still remains attached 

 to its pedicle. 



At the extremity of each of the four longi- 

 tudinal vessels a little tubercle now becomes 

 developed, which, as it becomes elongated, is 

 converted into a tentacle, or sometimes, as in 

 Eudendrimn, by its bifurcation, two tentacula 

 are formed from each tubercle. 



At this period of its development the young 

 Tubularia spontaneously detaches itself from 

 the parent stem, presenting at the moment of 

 its separation the appearance of a balloon, or 

 rather of a melon. ( Fig. 50. 1 , 2, 3.) Its contrac- 

 tions become more and more lively, and it is 

 by the aid of these movements that its sepa- 

 ration is effected. The two poles of its 

 globular body may be seen to approach each 

 other, and to separate alternately, with a 

 movement of systole and diastole similar to 

 what is observable in many Medusa?. No 

 traces of cilia are observable either externally 

 or in the interior of its body. In this con- 

 dition it presents an external covering, which 

 is, so to speak, merely a derivation from the 

 integument of the parent polype : this cover- 

 ing presents somewhat more consistence than 

 the internal parts, and is open in front. 



A second membrane lines the preceding 

 throughout its whole extent ; like the former, 

 it is quite transparent, and at the anterior 

 opening (c) is prolonged internally to a little 

 distance, forming a sort of funnel. These walls 

 enclose four vessels (c), which extend from the 

 base of the embryo and open in front into 

 the hollow zone (a), from which the tentacula 

 take their origin. These longitudinal vessels 

 therefore communicate with each other by 

 a transverse canal, and at their origin open 

 into the central or digestive cavity, which 



