70 



INVERTEBRATA 



CHAP. 



cavity. Each of these is produced by an inwardly-directed fold of 

 the endoderm, between the limbs of which is jelly.-- These taeniolae 

 are also termed septa (Tig. 48). 



Then a circular depression appears in the ectoderm on the upper 

 part of the larva, which marks off the oral cone from the bases of the 

 tentacles, and just above the upper ends of the four endodermal folds 

 this depression appears to be deeper. From the bottom of these 

 deeper depressions, which are termed the septal funnels, ectoderm 



cells are budded off and force 

 their way into the jelly between 

 the limbs of the taeniolar folds. 

 These cells develop fine mus- 

 cular fibrils on their external 

 surfaces, which form the four 

 longitudinal septal muscles. 

 These muscles extend down 

 to the base of the lar.va ; they 

 are exceedingly irritable and 

 serve to contract it (Fig. 49). 



The larva which is now 

 provided with a flattened upper 

 surface or oral disc, with four 

 long tentacles with solid axes, 

 with four endodermal septa, 

 and four ectoderinal septal 

 muscles, is termed a Hydra- 

 tuba or Scyphistoma. Four 

 secondary tentacles alternating 

 with the first four are soon 

 added, and eventually eight 

 tertiary ones, alternating with 

 the primary and secondary, so 

 that in all sixteen are formed. 



As Sir J. Dalyell (1847), 

 the first discoverer of this larva 

 showed, lateral buds like those 

 of a true Hydra can be formed, 

 which repeat the structure of the parent and eventually become 

 detached, and stolons can grow out from the body wall just above 

 the base, extending a short distance, and from them other hydra- 

 tubae can be given off. 



Friedemann (1902) takes up his account of the development 

 where Hein left off. The eight- tentacle d hydra-tuba grows in 

 size as it captures more and more prey. Then eight new 

 tentacles make their appearance alternating with the former, so 

 that the animal now possesses sixteen tentacles. The number is 

 then raised to twenty-four by the appearance of eight new ones, 

 and with this number the hydra -tuba attains the maximum of 



FIG. 48. Two transverse sections through a 

 Hydra- tuba with four tentacles. (After 

 Hein.) 



A, grazing the upper surface or oral disc. B, 

 through the middle of the body, s.p, stomach pocket ; 

 t.b, endoderm cells forming the bases of the tentacle ; 

 t.n, taeniolae. 



