ANATOMY AND DEVELOPMENT OF UYDUA. 15 



nervo-muscle cells. The interstitial tissue subsequently 

 shows itself between them in the form of irregular or fusi- 

 form cells. The embryo stretches itself out, and assumes an 

 ellipsoidal figure; its walls become thinned at one end, where 

 a stellate cleft suddenly makes its appearance. This is to 

 become the mouth. Simultaneously with the appearance of 

 the mouth the foundation of the tentacles has been laid in 

 the form of tubular offsets of the body cavity. That layer of 

 the body walls which is to form the endoderm remains as a 

 stratum of continuous protoplasm until after the formation of 

 the mouth, when we find it converted into a layer of pris- 

 matic cells. The thin inner shell which had continued to 

 invest the embryo after the destruction of the outer one now 

 becomes softened and dissolved, and the young animal is set 

 free, corresponding in all respects except in size with the 

 adult, all the tissues having differentiated themselves to their 

 definitive form, and even the thread-cells, though still few, 

 being already ripe for emission. 



From what we have thus seen of the development of 

 Hydra it is evident that this animal passes through no 

 proper larva stage. It differs in this from all other known 

 hydroids. We know that the egg of every other hydroid — 

 if we except that of the monopsean medusae — becomes 

 directly developed either like Sertularia into a planula, or 

 like Tubularia into an actinula. A certain resemblance 

 between the young Hydra and the young Tubularia induced 

 me to include the early free stage of both under the name of 

 actinula. As it appears now, however, that Hydra passes 

 from the condition in which it first becomes free to its adult 

 state by continuous growth without any true metamorphosis, 

 I accept, so far, the justice of Kleinenberg's criticism, and' 

 believe with him that the term actinula is not strictly appli- 

 cable to it. It is otherwise, however, with regard to Tubu- 

 laria. Here we have a true metamorphosis, and the young 

 is a free larva which undergoes important transformations 

 before it becomes converted into the fixed adult. The term 

 actinula is thus a convenient designation for the larva of 

 Tubularia and of the apparently similar larva of Myriothela, 

 and in this sense I have employed it in contradistinction to 

 the term planula, by which the earliest free stage of other 

 hydroids had been already known. 



I would accordingly distinguish three different modes in 

 the development of the hydriform trophosome : 



1. The development subsequently to the liberation of the 

 young animal takes place by continuous growth without any 

 metamorphosis. This mode exists in Hydra alone. 



