TEGUMENTARY ORGANS. 



Cainpanularia, for instance, the outer wall 

 of the bud from which a polype is to arise 

 consists, at first, of a mass of indifferent tissue. 

 As development proceeds, the outer portion 

 of the mass is converted into a structure- 

 less membrane, which becomes detached from 

 the body of the polype through its whole 

 extent, and constitutes the future cell, the 

 subjacent ecderon taking on the ordinary 

 cellular structure. On the pedicle the same 

 process goes on to a less extent, the struc- 

 tureless layer becoming separated only at 

 intervals, so that the pedicle acquires a ringed 

 appearance. 



An integument of one or other of these 

 descriptions is to be met with in all the 

 Sertularian and Actinoid Polypes, and is 

 obviously, in these cases, the result of a 

 process of excretion. In the Medusce and 

 Beroidce, on the other hand, where the integu- 

 ment is thick and gelatinous, the ecderonic 

 tissue is converted, as a whole, into what 

 closely resembles rudimentary connective 

 tissue, in which elastic elements and muscular 

 fibres are developed. The presence of peculiar 

 organs, called the " Thread or Urticating cells" 

 constitutes an extremely characteristic feature 

 in the integument of these creatures. These 



JFfe.311. 



(,fig. 311.) are composed of a delicate mem- 

 branous sac (), enclosing a much thicker one 

 (6), which is open at one extremity, the 

 aperture being stopped by the end of a more 

 or less irregular short stiff sheath (c), some- 

 times giving attachment to several distinct 

 rays or spines (rf), applied together, which is 

 fixed to the edges of the aperture, and oc- 

 cupies the axis of the inner sac. To the ex- 

 tremity of this sheath a long, frequently 

 toothed filament is attached (e), and lies 

 coiled up round the central sheath, and in 

 close contact with the walls of the sac. The 

 latter are very elastic, and seem to be tensely 

 stretched by the contained fluid during life ; 

 for, on pressure, the sac suddenly bursts, and 



its contents are evacuated so rapidlv as hardly 

 to allow of the process being traced. 1 be- 

 lieve, however, that the long filament is pushed 

 out by the side or through the axis of the 

 central sheath, remaining still firmly attached 

 to the latter, so that the result is the appear- 

 ance exhibited in the accompanying figure (c), 

 where the sac is seen empty, the long serrated 

 filament being attached to the sheath, which, 

 everted and with its spines spread out, is itself 

 fixed to the margins of the aperture. The 

 violent protrusions of these minute serrated 

 filaments, aided, perhaps, by some aridity of 

 the liquid of the sac, is in the larger kinds, 

 such as those which exist in Physalia, exceed- 

 ingly irritating to the human skin, and usually 

 proves fatal to the minute creatures on which 

 the Hydrozoic and Anthozoic polypes prey. 



Integument of the Annulosa. The integu- 

 ment of the lower Annulose tribes, of young 

 forms and of the more delicate parts of a 

 great majority of the higher Annulosa, con- 

 sists of a thin structureless chitinous mem- 

 brane developed from the subjacent cellular 

 ecderon, in a manner essentially similar to 

 what has been described in the Polypes. 



Leydighas particularly described this form 

 of integument in Entomostracous Crustaceans, 

 (Branchipus and Argulus) in insect larvae, 

 (Corethra), and among the Annelids in Pisci- 

 cola, Nephelis, Haemopis, Sanguisuga, Clep- 

 sine and Lumbricus, where the integument 

 consists of two portions a deep cellular layer 

 and a superficial layer, which is either abso- 

 lutely structureless, or is fibrillated ; being in 

 no case formed by the coalescence of the sub- 

 jacent cells, but by excretion from them. 



A similar structureless excreted integument 

 is found also in Planariae, Nemertidae, in many 

 Cestoidia, Nematoidea and Trematoda, and, 

 according to the late researches of Leydig, on 

 Synapta, in the Echinoderms also. Where 

 the integument is not very thin, and con- 

 sists of several layers of chitinous matter, the 

 added laminae commonly take on a fibrous 

 structure. The Nematoid worms present par- 

 ticularly good examples of this complication. 

 Thus, for instance, the integument of Mermis 

 albicans, which has lately been examined with 

 much care by Dr. Meissner, consists of three 

 layers, the middle of which is double. The 

 outermost of these layers is either structureless 

 or presents a distinction into transverse hex- 

 agonal plates, each of which occupies ^ of the 

 circumference of the animal. At the head 

 and tail, small polygonal plate-like markings 

 replace these, and such small plates could be 

 detected, making up the large ones. Dr. 

 Meissner calls them " cells," but expressly 

 states that he never detected any nucleus in 

 them, and it seems more probable that they 

 are produced by modifications of the original 

 external structureless layer, similar to those 

 which, as will be seen, occur in the Crustacea 

 and Mollusca. 



The middle substance of this integument is 

 composed of two layers of fibres one above 

 the other. The fibres are parallel in the same 

 laver, but those of the two layers cross one 



i i 3 



