76 



LOWER INVERTEBRATES. 



plete, it catches and digests food, and performs all its functions while still attached to 

 its parent. After a time a constriction separates it from its parent, but the opening at 

 its base never entirely closes (at least in some species), and is 

 known as the porus abdominalis. It does not function as an 

 anus, however, and cannot be so considered. Before the first 

 bud is set free, a second one may appear, and even a third and 

 fourth on the parent body. Moreover, a secondary bud may 

 appear on the body of the first bud, a tertiary on the body of 

 the second, and a fourth on the body of the third before the 

 first bud has become free. This is known as the compound or 

 colonial condition. 



Another method of increase which rarely occurs in Hydra 

 is division or fission, in which the entire animal divides into 

 two parts, each developing all the parts necessary to make it a 

 complete Hydra. Trembley observed this method, Rosel also 

 witnessed it, and Marshall has seen three cases of it. In this 

 country the process has been seen by Mr. T. B. Jennings, of 

 Springfield, 111. The wonderful power which Hydra possesses 

 of reproducing lost parts was first discovered and made known 

 by Trembley, of Geneva, in the first half of the eighteenth cen- 

 tury. He determined that even a small piece of Hydra vulgaris 

 possesses the power, under favorable conditions, of developing 

 into a perfect animal. His experiments were very varied, and 

 many of them have been often repeated with the same results, 

 since his day. Baker repeated nearly all 

 of them. The most remarkable of his 

 experiments in this line, was the turning 

 of the hollow, cylindrical body of a Hydra 

 inside out ; so that the inner layer which 

 before did the digesting, now performed 

 the functions of the cuticle, and vice 

 versa. This experiment, which requu-es 

 very skilful manipulation, has been, I 

 believe, repeated but by one biologist. 

 Professor Mitsukuri, of the University 

 of Tokio, Japan. 



In a limited region on the body of Hydra, just below the 

 tentacles, there appear under certain conditions, small out- 

 growths of the body-wall which prove to be the spermaries; 

 in them being developed the spermatozoa. Lower down on the 

 ^^°tion "of'^^So^reatiy ^°^y> ^^ another limited zone, larger, rounded swellings are 

 enlarged;^ o, ten taoi^^Bj^, ^.eveloped, which are the ovaries. Just how fertilization is 

 n, endoderm; m, mouth; accomplished is Unknown, but the esrs; bavins: been fertilized 



8, supporting lamella. ^ ' o° o 



passes through a morula stage in which the outer cells become 

 prismatic, forming a definite membrane around the interior ; a chitinous coat is devel- 

 oped about it, and then there occurs a retrograde step, as the entire embryo fuses into 

 a simple, non-cellular mass ; within this mass a small cavity appears, the first formation 

 of the body cavity. In this condition it remains quiescent for a time, and then the 



Fig. 67 — Transverse section 

 of Hydra, greatly en- 

 larged; letters as in fig. 

 66. 



