TEGUMENTARY ORGANS 389 



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 rapidly as hardly to allow 

 of the process being traced. I believe, 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 

 appearance 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 acidity of the liquid of the 

 sac, is in the larger kinds, such as those which exist in Physalia, 

 exceedingly 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 integument of the lower 

 Annulose tribes, of young forms and of the more delicate parts of a 

 great majority of the higher Annulosa, consists of a thin structureless 

 chitinous membrane developed from the subjacent cellular ecderon, 

 in a manner essentially similar to what has been described in the 

 Polypes. 



Leydig has particularly described this form of integument in 

 Entomostracous Crustaceans, (Branchipus and Argulus) in insect 

 larvs, (Corethra), and among the Annelids in Piscicola, Nephelis, 

 Haemopis, Sanguisuga, Clepsine and Lumbricus, where the integument 

 consists of two portions — a deep cellular layer and a superficial layer, 

 which is either absolutely structureless, or is fibrillated ; being in no 

 case formed by the coalescence of the subjacent cells, but by excretion 

 from them. 



A similar structureless excreted integument is found also in 

 Planarise, Nemertidse, 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 

 consists of several layers of chitinous matter, the added lamina 

 commonly take on a fibrous structure. The Nematoid worms present 

 particularly 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 structure- 

 less or presents a distinction into transverse hexagonal 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 



