22 



POLYPIFERA. 



to lose its distinctness of outline, and its parts 

 become dim and confused, for the process of 

 digestion has begun, and speedily all that is 

 digestible is dissolved, nothing being left but 

 the hard shell and other intractable portions, 

 which are at length expelled from the digestive 

 sac through the same opening by which they 

 were admitted. 



From the transparency of the Hydra, Trem- 

 bley thought to be able to ascertain the man- 

 ner in which the digested nutriment became 

 appropriated, and observing that the polypes 

 became coloured in accordance with the kind 

 of food upon which they lived, proceeded to 

 feed them with the red larvae of certain insects, 

 in hopes of seeing how the colouring matter 

 became diffused through their bodies, and in 

 this he was partially successful ; the result of 

 his experiments proving that it was through 

 the medium of the granules floating in the 

 semifluid transparent substance of the Hydra 

 that the diffusion of the coloured particles was 

 accomplished, the granules themselves assum- 

 ing the tint of the coloured food, while the 

 gelatinous matter in which they were sus- 

 pended remained colourless. 



Another remarkable fact observed by Trem- 

 bley was, that the digestive powers of the 

 Hydra had no influence over the tissues of its 

 own body, for frequently he observed that the 

 long-armed species swallowed their own ten- 

 tacula along with their food, the former re- 

 maining quite intact while the latter was in 

 process of solution, and on one occasion 

 when two Hydras had both of them seized on 

 the same prey, and were contending for the 

 possession of it, one of them decided the con- 

 test by swallowing the subject of dispute and 

 his rival into the bargain. Naturally supposing 

 that the death of the swallowed polype would 

 be the result of such an apparently tragical 

 termination to the dispute, Trembley was not 

 a little surprised to see the successful polype 

 disgorge his antagonist safe and uninjured 

 along with the egestainenta of the meal, and 

 to all appearance none the worse for its tem- 

 porary incarceration. 



If a Hydra be divided transversely, by means 

 of a knife or a pair of scissors, both halves not 

 only survive, but in the course of a short time 

 each moiety reproduces the portion of which 

 it has been deprived, the hinder extremity 

 developing a new set of tentacula, and the an- 

 terior portion acquiring a sucker to replace 

 that which was lost ; nay, it has been proved 

 that even when divided into several fragments, 

 eich piece retains its vitality, and in process 

 of time regains all the characters of a perfect 

 individual, just as the cutting of a plant 

 speedily puts forth roots and leaves similar to 

 those of the original stock from which it was 

 taken. 



Not less wonderful than their capability of 

 recovering lost parts after mutilation are the 

 powers which they possess of multiplying their 

 species by various modes of generation. The 

 most usual manner in which they produce off- 

 spring is by gemmation, the nature of which, 

 owing to the transparency of their bodies, they 



are admirably adapted to elucidate. The process 

 by which this kind of reproduction is effected in 

 the case of the Hydra is as follows. After 

 keeping one of these polypes for a few hours 

 well provided with food, a little bud or gemma 

 is seen to sprout from some portion of the 

 surface of its body, which at first seems to be 

 a shapeless excrescence, but in the course of a 

 short time assumes the shape of the parent 

 animal by developing tentacula from around 

 the oral orifice, which gradually becomes more 

 and more distinct. For some time the newly 

 formed polype remains attached by the little 

 pedicle at its tail to the body of its parent, 

 with which it seems to enjoy a sort of com- 

 munity of existence, the food caught and di- 

 gested by the one passing freely through a 

 little aperture in the caudal extremity of the 

 young polype from one to the other. At last, 

 when the growth of the off-sprout is com- 

 pleted, it detaches itself, and assumes an in- 

 dependent existence ; yet sometimes even 

 before its separation is accomplished the bud 

 of a third generation may be observed ap- 

 pended to the side of its body ready to under- 

 go the same process of development. 



The formation of the reproductive gemmae 

 may even be determined by extraneous causes : 

 thus Trembley noticed that by snipping the 

 side of an adult polype with the points of a fine 

 pair of scissors, a bud would soon develope 

 itself from the wounded part ; and this experi- 

 ment might be repeated again and again, both 

 upon the original polype and the progeny thus 

 made to sprout from its sides, until as many 

 as seventeen have been obtained, all con- 

 nected with each other, and thus forming a 

 little tree of living polypes. 



Besides the gemmiparous mode of repro- 

 duction, Hydrae have been occasionally ob- 

 served to divide themselves spontaneously by 

 transverse fissure, and thus separate into two 

 animals, in the same way as some of the Poly- 

 gastric animalcules. 



The anatomy of the Hydra has been recently 

 closely investigated by Corda, whose obser- 

 vations upon this subject are possessed of 

 extreme interest.* According to this observer 

 each tentacule of the Hydra consists essentially 

 of a long, pellucid, and extremely delicate 

 membranous tube (fig. 27) containing an al- 

 most fluid albuminous substance, which in 

 certain definite localities swells into denser 

 wart-like knots (6), arranged in a spiral line, 

 along which are appended organs of touch (d), 

 and also instruments of prehension (c). Si- 

 tuated within the tube, and running imme- 

 diately beneath the above-mentioned nodosities, 

 which are arranged in a quaternian series, are 

 situated four longitudinal bands of muscular 

 fibres of a slightly yellow colour (c), which 

 seem to be the extensors of the tentacles. 



These extensors of the tentacula are more- 

 over united to each other by transverse mus- 

 cular fasciculi ( /") of the same colour as them- 

 selves, which Corda names adductors of the 



* Anatome Hydras fnscre exposuit Augustus 

 Josephus Corda, cum tnlmlis tribus. Acta acad. 

 Gees. Leopold. Carol, naturte curiosomm, vol. xviii. 



